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Marine  Biological  Laboratory  Library 

Woods  Hole,  Mass. 

Presented  by 

January  9,  1964 














By  JACQUES  LOEB.  Rockefeller  Institute 


By  G.  H.  PARKER,  Harvard  University 


By  T.  H.  MORGAN.  Columbia  University 


By  B.  M.  EAST  and  D.  F.  JONES.  Bussey  Institution.  Harvard  University 


By  E.  N.  HARVEY.  Princeton  University 


By  G.  H.  PARKER.  Harvard  University 


By  R.  PEARL.  Johns  Hopkins  University 


By  H.  S.  JENNINGS.  Johns  Hopkins  University 



By  E.  G.  CONKLIN,  Princeton  University 


By  R.  G.  HARRISON.  Yale  University 


By  W.  J.  V.  OSTERHOUT.  Harvard  University 


By  L.  J.  HENDERSON.  Harvard  University 


By  T.  B.  ROBERTSON.  University  of  Toronto 


By  A.  R.  MOORE.  Rutgers  College 





G.  H.  PARKER,  Sc.D. 





Electrotypcd  and  Printed  by  J.  B.  Li ppincott  Company 
The   Washington  Square  Press,  Philadelphia,  U.  S.  A. 


THE  rapid  increase  of  specialization  makes  it  im- 
possible for  one  author  to  cover  satisfactorily  the  whole 
field  of  modern  Biology.  This  situation,  which  exists  in 
all  the  sciences,  has  induced  English  authors  to  issue 
series  of  monographs  in  Biochemistry,  Physiology,  and 
Physics.  A  number  of  American  biologists  have  decided 
to  provide  the  same  opportunity  for  the  study  of 
Experimental  Biology. 

Biology,  which  not  long  ago  was  purely  descriptive 
and  speculative,  has  begun  to  adopt  the  methods  of  the 
exact  sciences,  recognizing  that  for  permanent  progress 
not  only  experiments  are  required  but  quantitative  experi- 
ments. It  will  be  the  purpose  of  this  series  of  monographs 
to  emphasize  and  further  as  much  as  possible  this  develop- 
ment of  Biology. 

Experimental  Biology  and  General  Physiology  are  one 
and  the  same  science,  in  method  as  well  as  content,  since 
both  aim  at  explaining  life  from  the  physico-chemical 
constitution  of  living  matter.  The  series  of  monographs 
on  Experimental  Biology  will  therefore  include  the  field 
of  traditional  General  Physiology. 

T.  H.  MORGAN, 



SENSE  organs  have  always  excited  general  interest, 
for  they  are  the  means  of  approach  to  the  human  mind. 
Without  them  our  intellectual  life  would  be  a  blank.  The 
deaf  and  the  blind  show  how  serious  is  the  loss  of  even  a 
single  set  of  these  organs. 

Although  the  ear  and  the  eye  have  commonly  received 
most  attention,  the  other  sense  organs,  such  as  those  of 
smell  and  of  taste,  are  in  reality  equally  worthy  of  con- 
sideration. These  organs  are  of  first  significance  in 
warning  us  of  untoward  conditions  that  may  exist  about 
us  particularly  in  relation  to  our  food.  But  they  not  only 
serve  us  in  this  protective  way,  they  are  also  of  the  utmost 
importance  in  initiating  that  chain  of  events  which  cul- 
minates in  successful  nutrition.  Through  their  action  the 
secretion  of  the  digestive  juices  and  other  like  operations, 
so  essential  to  the  proper  treatment  of  the  food,  are 
started  and  furthered  in  the  alimentary  canal.  Thus 
their  activities,  though  less  associated  with  our  mental 
states  than  are  those  of  the  ear  and  of  the  eye,  are  never- 
theless so  essential  to  our  organic  well-being  that  they 
are  in  reality  quite  as  necessary  to  us  as  the  so-called 
higher  senses. 

Smell  and  taste,  together  with  certain  other  senses  not 
so  well  known,  form  a  more  or  less  natural  group  in  which 
there  is  a  certain  amount  of  functional  interrelation  and 
genetic  connection,  and  it  is  from  this  standpoint  that 
these  senses  will  be  considered  in  the  following  pages. 
They  will  thus  illustrate  in  a  way  principles  common  to 



other  groups  of  sense  organs,  and  these  principles  will  be 
found  to  be  of  an  essentially  dynamic  character  as  con- 
trasted with  the  older  conceptions  in  which  function  has 
been  brought  into  relation  less  intimately  with  structure. 

The  author  is  greatly  indebted  to  the  editors  of  this 
series  of  monographs  for  many  suggestions  that  have  led 
to  improvements  in  the  text.  He  is  also  under  obligations 
to  his  wife  for  a  careful  revision  of  the  manuscript.  He 
wishes  to  extend  his  thanks  to  numerous  persons  who 
have  permitted  him  to  copy  and  use  figures  contained  in 
their  publications.  In  all  such  instances  the  sources  of 
such  figures  are  acknowledged  in  the  text.  Where  a 
figure  is  given  without  reference,  it  is  an  original.  The 
drawings  for  all  figures  were  made  by  Mr.  E.  N.  Fisher. 

G.  H.  P. 
Harvard  University,  Cambridge,  Mass. 

January,  1922. 











INDEX  .  187 

R  ?  •  -1 8 "! 

Q  fat    .$.   ^J  Ju 



1.  Diagram  of  the  Lateral  Wall  of  the  Right  Nasal  Cavity  of  Man . .  24 

2.  Diagram  of  a  Transverse  Section  of  the  Right  Nasal  Cavity  of  Man  25 

3.  Respiratory  Epithelium  from  the  Nasal  Cavity  of  a  Young  Pig. ...  27 

4.  Olfactory  Cleft  of  Man 28 

5.  Olfactory  Epithelium  from  a  Pig  Embryo 29 

6.  Olfactory  Epithelium  from  a  Young  Mouse 30 

7.  Isolated   Olfactory  Cells  and  Sustentacular  Cells  from  Man 31 

8.  Isolated  Olfactory  Cell  and  Sustentacular  Cell  from  a  Frog 32 

9.  Olfactory  Cell  of  a  Pike  Showing  Flagellum 33 

10.  Olfactory  Epithelium  from  a  Chick  Embryo 36 

11.  Ventral  View  of  the  Head  of  a  Shark  (Scyllium) 38 

12.  Diagram  of  the  Right  Nasal  Cavity  of  Man  Showing  the  Direction 

of  the  Inspired  Air  Currents 46 

13.  Simple  Rubber  Olfactometer 50 

14.  Double  Olfactometer 51 

15.  Ventral  View  of  the  Head  of  a  Hammer-head  Shark 66 

16.  Curves  of  Olfactory  Exhaustion 71 

17.  Olfactory  Prism 75 

18.  Generalized    Diagrams    of    the    Molecular    Structure     of      Classes 

of  Aromatic  Bodies  (Olfactory  Stimuli) 80 

19.  Head  of  Human  Embryo  showing  Vomero-nasal  Pore 93 

20.  Diagram  of  the  Median  Face  of  the  Left  Nasal  Cavity  of  Man 94 

21.  Transverse  Section  of  the  Snout  of  a  Young  Frog 95 

22.  Transverse  Section  of  the  Head  of  a  Snake  Embryo 96 

23.  Transverse  Section  of  the  Nasal  Septum  of  a  Young  Cat 97 

24.  Epithelium  from  the  Vomero-nasal  Organ  of  the  Sheep 98 



25.  Dorsal  View  of  the  Human  Tongue 112 

26.  Vertical  Section  of  a  Fungiforra  Papilla 113 

27.  Vertical  Section  of  a  Vallate  Papilla 114 

28.  Lateral    View  of    a   Catfish  Showing  Gustatory  Branches  of    the 

Facial  Nerve 116 

29.  A  Simple  Taste-bud 117 

30.  A  Compound  Taste-bud 118 

31.  Taste-buds  of  the  Rabbit 121 

32.  Taste-buds  of  the  Cat 122 

33.  Taste-buds  of  the  European  Barbel 124 

34.  Diagram  of  the  Human  Tongue  Showing  Innervation 125 

35.  Diagram  of  the  Possible  Paths  of  the  Gustatory  Nerves  in  Man.  . . .  126 

36.  Diagrams  of  the  Human  Tongue  Showing  the  Distribution  of  the 

Four  Tastes 149 

37.  Diagrams  of  the  Receptor  Systems  of  the  Vertebrate  Chemoreceptors  181 




Contents. — 1.  Older  Conception  of  Sense  Organs.  2. 
Modified  View  due  to  Theory  of  Reflex  Action.  3.  The 
Genesis  of  Receptors.  4.  Bibliography. 

1.  OLDER  Conception  of  Sense  Organs.  In  the  con- 
ventional text-book,  sense  organs  are  commonly  looked 
upon  as  structures  that  supply  the  brain  with  those  nerv- 
ous impressions  from  which  the  mental  life  of  the  indi- 
vidual is  built.  During  normal  activity  these  organs  are 
incessantly  in  operation  and  flood  the  central  apparatus 
with  a  stream  of  impulses  by  which  are  carried  to  us  evi- 
dences of  the  multitudinous  alterations  of  the  environ- 
ment. Through  the  ear  and  the  eye  pass  continuous 
streams  of  change  by  which  we  adjust  ourselves  not  only 
to  the  immediate  material  world  about  us  but  to  the 
world  of  ideas  whose  elements  are  spoken  and  writ- 
ten words. 

Sense  organs  from  a  structural  standpoint  are  organs 
whose  cells  are  so  specialized  that  they  are  subject  to  stim- 
ulation by  only  a  particular  category  of  external  changes. 
As  Keith  Lucas  has  expressed  it,  sense  cells  approximate 
a  unifunctional  state.  The  changes  by  which  they  are 
brought  into  action  form  rather  homogeneous  groups  of 



environmental  alterations.  Thus  the  chemical  changes  of 
the  surroundings  affect  the  organs  of  smell  and  of  taste, 
the  pressure  changes  those  of  touch  and  hearing,  and  al- 
terations in  the  radiant  energy  those  of  sight.  These 
natural  groups  of  environmental  changes  have  been  des- 
ignated as  homologous,  or,  better,  adequate  stimuli  for 
the  sense  organ  that  they  activate.  Such  organs  are  ordi- 
narily arranged  under  five  heads  each  with  an  adequate 
stimulus  and  productive  of  a  special  sensation ;  they  are 
the  organs  of  smell,  taste,  touch,  hearing,  and  sight. 

Experience  has  also  shown  that  when  in  a  given  per- 
son a  sense  organ  exhibits  complete  congenital  incapacity, 
such  an  individual  lacks  certain  mental  elements  that  can 
never  in  reality  be  made  good  to  him  by  the  activity  of  the 
remaining  parts.  A  state  of  this  kind  implies  a  certain 
mental  deficiency  in  the  given  individual.  If  a  person  has 
been  blind  from  birth,  no  amount  of  description  can 
supply  to  him  the  sensations  of  the  wealth  of  color  that  the 
external  world  holds  for  the  normal  man.  Where  blind- 
ness is  an  acquired  defect,  the  rememberance  of  the 
former  color  sensations  as  compared  with  the  present 
deprivation,  makes  the  state  of  deficiency  still  more  pro- 
nounced. And  in  those  rare  cases  where  there  is  a 
unilateral  defect  in  color  vision  with  sight  otherwise  unim- 
paired, the  subject  can  contrast  most  vividly  the  state  of 
deficiency  with  that  of  normal  completeness.  Such  con- 
ditions, which  are  known  to  occur  not  only  in  sight  but  in 
the  other  senses  as  well,  have  had  a  most  profound  influ- 
ence on  the  interpretations  that  naturalists  have  placed 
upon  the  states  presented  by  the  lower  animals. 

It  has  been  commonly  assumed,  and  with  no  small 
show  of  reason,  that  where  an  animal  is  found  to  possess 


an  eye  or  an  ear,  for  instance,  it  should  be  accredited  with 
all  the  central  nervous  activities,  sensations  and  the  like, 
that  accompany  such  an  organ  in  man,  qualified  only  by 
the  degree  of  development  to  which  the  particular  organ 
in  the  given  animal  has  arrived.  Conclusions  based 
upon  such  a  course  of  reasoning  were  commonly  ad- 
mitted as  valid  by  the  workers  of  a  few  decades  ago 
(Lubbock,  1882;  Graber,  1884)  and  the  text-books  of  that 
period  in  dealing  with  the  sense  organs  of  the  lower 
animals  discuss  these  parts  ordinarily  under  the  conven- 
tional five  heads  of  the  older  human  physiology  ( Jourdan, 
1889).  From  this  standpoint  one  of  the  lower  animals  is 
like  a  defective  human  being  in  that  its  full  sensory  ac- 
tivity falls  short  of  that  of  the  normal  man.  Or  it  may  be 
compared  to  a  person  whose  sensory  development  is  un- 
symmetrical  and  whose  relations  with  the  surroundings 
have  come  to  be  predominant  through  a  limited  number 
of  sensory  channels  rather  than  through  all. 

It  is  likewise  perfectly  clear  that  a  given  animal,  whose 
organization  in  general  may  be  simpler  than  that  of  man, 
may  nevertheless  excede  him  in  a  particular  sensory 
capacity  and  in  this  respect  at  least  stand  above  him.  It  is 
commonly  admitted  that  the  dog  far  outruns  man  in  the 
keeness  of  Ms  sense  of  smell  and  it  has  long  been  known 
that  cats  hear  tones  of  a  pitch  much  too  high  for  the  human 
ear.  These  and  other  like  examples  show  that  though  the 
senses  of  the  lower  animals  are  in  general  less  efficient 
than  those  of  man,  the  reverse  is  occasionally  true. 

Moreover  among  some  of  the  lower  forms,  sense  or- 
gans have  been  discovered  that  are  not  represented  in 
man.  Thus  fishes  possess,  in  addition  to  the  five  classes  of 
human  sense  organs,  the  so-called  lateral-line  organs. 


Here  then  must  be  a  wholly  novel  set  of  sensory  relations. 
As  to  the  sensations  arising  from  these  organs  man  can 
form  no  direct  conception,  for  they  are  entirely  outside  the 
range  of  his  experience.  Hence  Leydig,  the  discoverer  of 
the  sensory  nature  of  these  parts,  wrote  of  them  as  organs 
of  a  sixth  sense.  Thus  to  the  older  workers  the  senses  of 
the  lower  animals  were  like  those  of  a  human  being  that 
had  suffered  either  curtailment  or  expansion  even  to  the 
extent  of  excluding  or  including  whole  categories  of 
stimuli.  But  quite  aside  from  the  question  of  the  number 
and  variety  of  these  parts,  is  the  opinion  held  by  most  of 
the  early  workers  that  the  sense  organs  of  the  lower  ani- 
mals are  primarily  concerned  with  providing  the  brain 
or  corresponding  structure  of  the  given  creature  with  that 
body  of  sensation  which  was  supposed  to  represent  all  the 
significant  changes  in  the  effective  environment. 

2.  Modified  View  due  to  Theory  of  Reflex  Action. 
The  belief  that  sense  organs  were  chiefly  concerned  with 
providing  the  brain  with  the  elements  of  which  the  mental 
life  is  composed  suffered  an  important  limitation  from  the 
work  of  the  physiologist.  This  limitation  arose  from  the 
development  of  the  idea  of  reflex  action.  Originating 
about  the  time  of  Descartes  in  the  seventeenth  century,  the 
conception  of  the  reflex  action  grew  in  time  into  a  most 
important  principle  for  the  interpretation  of  nervous 
operations.  It  was  at  first  applied  to  that  form  of 
nervous  activity  whose  outcome  is  fairly  constant  and  in 
a  way  mechanical  in  that  it  is  unassociated  with  conscious- 
ness, but  it  was  gradually  extended  to  include  those  per- 
formances in  which  consciousness  is  involved  and  at 
present  it  commonly  refers  to  any  chain  of  nervous 
activity  in  which  a  sensory  stimulation  produces  an  im- 


pulse  that,  after  passage  through  the  central  nervous 
organs,  results  in  action. 

From  the  beginning  many  reflexes  were  believed  to  be 
unassociated  with  consciousness  and  though  this  view  was 
subsequently  combated  and  the  idea  of  the  reflex  extended 
to  nervous  operations  that  included  an  obvious  sensa- 
tional element,  it  nevertheless  remained  true  that  a  host 
of  reflex  operations  could  be  pointed  out  that  were  with- 
out representation  in  consciousness.  Thus  the  impulses 
that  flow  from  the  vestibular  portion  of  the  human  ear 
and  that  are  of  the  utmost  importance  in  maintaining 
equilibrium  provoke  no  obvious  sensations  and  the  vast 
flux  of  afferent  nerve  action  that  moves  from  the  mus- 
cle to  the  spinal  cord  and  that  is  so  essential  to  the 
coordination  of  bodily  movements,  runs  its  course  without 
exciting  sensation.  These  and  many  like  instances  have 
made  it  clear  that  the  reflex,  even  in  the  most  special  ap- 
plication of  the  term  may  as  often  be  unassociated  with 
sensation  as  associated  with  it. 

As  the  first  step  in  every  reflex  is  the  excitation  of  a 
sense  organ  and  as  many  reflexes  are  unassociated  with 
consciousness,  it  must  be  admitted  that  sense  organs,  not- 
withstanding the  name,  are  not  always  necessarily  con- 
cerned with  sensations.  Many  certainly  have  nothing 
whatever  to  do  with  such  central  nervous  states.  Thus 
it  is  doubtful  if  the  normal  activity  of  the  sensory  endings 
in  our  muscles  and  tendons  is  ever  productive  of  sensation. 
In  consequence  of  this  condition  a  reasonable  objection 
was  raised  to  the  term  sense  organ  and  it  was  proposed  by 
Bethe  (1897)  to  use  in  place  of  it  the  word  receptor. 
Although  the  theoretic  force  of  this  objection  has  not 
always  carried  conviction,  the  term  receptor  has  come  into 



common  use  and  the  emphasis  that  it  places  on  the  organs 
to  which  it  is  applied  as  receivers  of  environmental  change 
rather  than  as  originators  of  impulses  to  sensation  is 
certainly  a  step  in  the  right  direction. 

Human  receptors  belong  to  one  or  other  of  two  classes. 
Either  they  are  concerned  purely  and  simply  with  the 
excitation  of  reflex  acts  and  take  no  part  in  the  pro- 
duction of  sensations,  in  which  case  they  may  be  called 
activators,  or  they  are  at  the  same  time  effective  in 
arousing  sensations,  the  elements  of  the  intellectual  life 
and  hence  may  be  appropriately  termed  sense  organs. 
All  receptors  belong  to  either  one  or  the  other  of  these 
classes  though  in  some  instances  a  certain  degree  of 
temporary  vacillation  occurs.  Hence  it  may  be  that  these 
classes  exemplify  in  a  way  two  receptive  functions,  one 
of  which  predominates  in  one  class  and  the  other  in  the 
other.  How  these  functions  are  related  can  best  be 
gathered  from  the  genetic  history  of  receptors. 

3.  The  Genesis  of  Receptors.  Eeceptors  such  as  the  eye 
and  the  ear,  the  organs  of  smell  and  taste,  and  the  more 
diffuse  sensory  equipment  of  the  skin,  are  found  in  all 
the  more  complex  animals.  They  abound  in  the  verte- 
brates, the  mollusks,the  arthropods,  and  to  a  less  extent  in 
the  worms.  They  may  be  said  to  occur  even  in  the  coe- 
lenterates,  as,  for  instance,  among  the  jelly  fishes,  though 
in  the  majority  of  these  animals  the  receptors  present  a 
diffuse  condition  more  like  that  seen  in  the  vertebrate 
skin  than  in  the  vertebrate  eye  or  ear.  This  diffuse  state 
seems  to  be  characteristic  of  the  receptors  in  the  simpler 
sessile  invertebrates.  The  more  complex  animals  such  as 
are  capable  of  active  locomotion  exhibit  almost  invari- 
ably specialized  types  of  organs. 


So  far  as  the  neuromuscular  system  of  the  inverte- 
brates is  concerned,  forms  as  low  in  the  scale  as  the  annelid 
worms  appear  to  possess  all  the  elements  of  the  corre- 
sponding system  in  the  vertebrates.  Such  worms  may 
have  specialized  receptors,  eyes  and  the  like,  often  of  a 
highly  complex  structure.  They  possess  a  well-differ- 
entiated central  nervous  system  as  represented  in  their 
so-called  brain  and  ventral  ganglionic  chain.  Finally, 
they  have  an  abundant  variety  of  specialized  effectors 
in  their  various  muscles,  glands,  and  luminous  organs. 
Their  receptors,  central  nervous  organs,  and  muscles  are 
so  related  that  reflexes  can  be  demonstrated  on  them  as 
readily  as  on  vertebrate  preparations.  In  other  words, 
they  possess  in  completeness,  though  in  simple  form,  a 
working  neuromuscular  mechanism  essentially  like  that 
of  the  higher  animals. 

When,  however,  an  examination  of  such  forms  as  the 
ccelenterates  is  made,  it  is  found  that  the  coral  animals, 
the  sea-anemones,  the  hydroids,  and  the  like,  possess 
scarcely  any  trace  of  a  central  nervous  apparatus.  In 
these  animals  fairly  well  specialized  sensory  surfaces 
occur,  whose  nervous  prolongations  connect  either  imme- 
diately with  the  subjacent  musculature  or  give  rise  to  a 
nerve-net  which  in  turn  connects  with  the  contractile  ele- 
ments. Thus  the  receptor  is  applied  to  the  muscle  very 
directly  and  without  the  intervention  of  a  central  organ. 
Such  an  arrangement  allows  of  simple  reflexes,  for,  when 
the  receptive  surface  is  stimulated,  the  animal  responds 
at  once  by  an  appropriate  muscular  movement.  Thus  if 
meat  juice  is  discharged  on  the  tentacles  of  a  sea-anemone, 
these  organs  carry  out  vermiculate  movements  and  the 
gullet  opens; or  if  the  pedal  edge  of  the  column  is  touched, 


the  whole  animal  contracts.  The  fact  that  meat  juice 
will  not  excite  the  pedal  edge  of  the  column  and  that  a 
touch  applied  to  the  tentacles  is  seldom  followed  by  more 
than  a  slight  local  activity  shows  that  the  external  surface 
of  the  sea-anemone,  though  generally  receptive,  is  locally 
specialized.  As  a  matter  of  fact  this  surface  in  degree  of 
differentiation  stands  between  a  diffuse  receptive  surface, 
such  as  the  vertebrate  skin,  and  a  specialized  organ  like 
the  eye  or  the  ear. 

In  the  literal  sense  of  the  word  the  outer  surface  of  a 
sea-anemone  is  not  sensory  though  abundantly  receptive. 
There  is  no  reason  to  suppose  that  the  receptive  areas  of 
these  animals  are  concerned  with  initiating  impulses  to 
sensation.  They  connect  very  directly  with  muscles  and 
serve  quite  obviously  as  trigger-like  organs  by  which  the 
muscle  is  set  in  action.  A  careful  examination  of  the 
activities  of  sea-anemones  has  failed  to  reveal  any  evi- 
dence, such  as  can  be  produced  from  the  more  complex 
animals,  to  show  that  these  simple  creatures  possess 
central  nervous  functions.  Such  functions  apparently 
have  no  part  in  their  organized  performances.  Hence 
their  receptors  have  nothing  whatever  to  do  with  initia- 
ting impulses  to  sensation,  but  are  limited  in  their  action 
to  the  excitation  of  the  muscles  after  the  type  of  the  most 
mechanical  reflex.  The  presence  in  ccelentrates  of  eye 
spots,  olfactory  pits,  statocysts  and  other  such  special 
receptors  is,  therefore,  no  indication  that  these  animals  are 
endowed  with  corresponding  sensations,  as  many  of  the 
older  workers  believed,  but  this  condition  merely  shows 
that  their  possessors  are  especially  open  to  a  particular 
stimulus.  An  eye  spot  does  not  mean  that  the  animal  pos- 
sesses sight,  but  that  it  is  readily  excited  to  action  by  light. 


Thus  of  the  two  functions  that  have  been  attributed  to 
receptors,  the  capacity  to  excite  action  and  the  ability  to 
initiate  impulses  for  sensation,  the  former  is  much  the 
more  widely  distributed  of  the  two  and  is  without  question 
the  more  primitive. 

Since  sponges  are  known  to  possess  muscles  but  are 
devoid  of  nervous  tissue,  it  is  probable  that  they  represent 
a  type  of  organization  which  in  point  of  time  preceded 
that  in  which  the  nervous  elements  arose.  So  far  as  can 
be  judged  these  elements  originated  in  connection  with  the 
previously  differentiated  muscle  and  as  a  special  means 
of  exciting  it  to  contraction.  This  earliest  nervous  mate- 
rial must  have  been,  therefore,  essentially  receptive  in 
character  and  must  have  served  as  the  source  of  the  more 
obvious  receptors  of  specialized  types.  Thus  receptors 
must  be  regarded  as  the  original  form  of  nervous  struc- 
ture, concerned  in  the  beginning  with  the  simple  excita- 
tion of  muscle  (activators)  and  subsequently  involved, 
after  the  development  of  the  central  organs,  with  that 
supply  of  impulses  which  yields  the  elements  of  the  intel- 
lectual life  (sense  organs). 

The  extent  to  which  a  natural  group  of  receptors  may 
undergo  differentiation  and  yet  maintain  a  striking  degree 
of  mutual  interdependence  can  nowhere  be  better  illus- 
trated than  with  the  chemical  receptors,  the  organs  of 
smell  and  of  taste.  It  is  from  this  standpoint  that  the 
structure  and  function  of  these  receptors  will  be  con- 
sidered in  the  following  chapters. 


BEER,  T.,  A.  BETHE,  und  J.  VON  UEXKULL.     1899.     Vorschlage  zu  einer 
objektivierenden  Nomenklatur  in  der  Physiologic  des  Nervensy steins. 
Biol.  Centralbl,  Bd.  19,  pp.  517-521. 


BETHE,   A.    1897.     Das  Nervensystem   von   Carcinus   maenas.     Arch.   mik. 

Anat.,  Bd.  50,  pp.  460-546. 
GBABEB.   V.     1884.       Grundlinien   zur    Erforschung    des    Helligkeits-   und 

Farbensinnes  der  Tiere.     Prag  &  Leipzig,  322  pp. 

JOURDAJST,  E.     1889.    Les  sens  chez  les  animaux  inferieurs.     Paris,  314  pp. 
LUBBOCK,  J.     1882.    Ants,  Bees,  and  Wasps.    New  York,  448  pp. 
PABKEB,  G.  H.  1910.     The  Reactions  of  Sponges,  with  a  Consideration  of  the 

Origin   of   the   Nervous   System.     Jour.  Exp.  Zool.,   vol.,   8,  pp.    1-41. 
PABKEB,  G.  R.  1917.     The  Sources  of  Nervous  Activity.     Science,  vol.  45, 

pp.  619-626. 
PABKEB,   G.   H.     1919.     The    Elementary   Nervous   System.     Philadelphia, 

229  pp. 



Contents. — 1.  Nasal  Cavities  in  Man.  2.  Nasal  Mem- 
branes. 3.  Olfactory  Epithelium.  4.  Intermediate  Zone. 
5.  Polymorphic  Cells.  6.  Sense  Buds.  7.  Free-nerve 
Endings.  8.  Development  of  Olfactory  Nerve.  9.  Com- 
parative Anatomy  of  Olfactory  Organs.  10.  Bibliography. 

1.  NASAL  Cavities  in  Man.  In  man  the  olfactory 
organs  are  paired  and  are  situated  one  in  each  nasal 
cavity.  Each  of  these  cavities  possesses  an  external 
opening,  the  anterior  nans,  and  an  internal  one,  the 
posterior  naris  or  choana,  which  communicates  with  the 
pharynx.  (Fig.  1).  The  two  nasal  cavities  are  separated 
by  the  nasal  septum,  a  partly  bony,  partly  cartilaginous 
wall,  which  forms  a  smooth  median  partition  between 
them.  The  lateral  walls  of  these  cavities  are  thrown  into 
a  series  of  more  or  less  horizontal  folds,  the  nasal  conchas. 
These  are  commonly  three  in  number  for  each  cavity 
though  in  some  instances  only  two  are  present  and  in 
others  a  fourth,  fifth  or  even  a  sixth  can  be  discerned. 
Of  the  three  conchas  usually  present  the  most  ventral  one, 
the  inferior  concha,  is  the  largest  and  extends  through 
much  of  the  length  of  the  cavity  in  a  direction  approxi- 
mately parallel  to  its  floor.  Immediately  above  the  inferior 
concha  is  the  somewhat  smaller  middle  concha  which 
is  followed  by  the  still  smaller  superior  concha.  When 
only  two  conchas  are  present,  they  are  the  inferior  and  the 



middle,  the  superior  being  absent.  When  a  fourth  concha 
is  to  be  seen,  it  is  found  above  and  behind  the  superior. 
It  has  been  designated  the  first  supreme  concha  and  it 

Fio.  1. — Diagram  of  the  lateral  wall  of  the  right  nasal  cavity  of  man.  I,  Inferior  concha; 
2,  middle  concha;  3,  superior  concha;  4,  first  supreme  concha;  5,  second  supreme  concha;  the 
apertures  numbered  C  to  10  arc  covered  from  sight  by  the  conchro,  but  their  positions  are 
indicated  by  vertical  lining;  6,  aperture  of  the  nnsolacrimal  duct  opening  into  the  inferior 
meatus;  7,  opening  of  the  maxillary  sinus  (middle  meatus);  8,  opening  of  the  frontal  sinus 
(middle  meatus);  'J,  and  10,  openings  of  the  ethmoid  cells,  9.  into  the  middle  meatus,  10, 
into  the  superior  meatus;  11,  opening  of  the  Kustachian  tube;  12,  vestibule;  13,  atrium;  14, 
choana;  15,  frontal  sinus;  10,  sphenoidal  sinus  whose  opening  is  indicated  by  an  arrow;  17, 
olfactory  region  whose  limits  are  marked  by  the  dotted  line.  The  vertical  dotted  line  shows 
the  plane  of  section  from  which  Fig.  2  was  drawn. 

may  be  followed  by  a  second  or  even,  a  third  supreme 
concha.  According  to  Schaeffer(1920),  the  first  supreme 
concha  is  to  be  observed  in  about  60  per  cent  of  all  adult 
human  beings. 

The  three  conchae  ordinarily  present  project  from 


the  lateral  wall  of  each  nasal  chamber  into  its  cavity  and 
partly  divide  that  cavity  into  three  approximately  hori- 
zontal passages:  the  inferior  meatus  under  the  inferior 
concha,  the  middle  meatus  under  the  middle  concha  and 
the  superior  meatus  under  the  superior  concha.  (Fig.  2). 
The  external  naris  leads  at  once 
to  the  first  chamber  of  the  nose, 
the  vestibule,  which  connects 
almost  directly  with  the  inferior 
meatus,  less  directly  with  the  su- 
perior meatus  and  through  the 
so-called  atrium  with  the  middle 
meatus.  Between  the  median  sep- 
tum of  the  nose  and  the  laterally 
situated  conchas  is  a  considerable 
space  known  as  the  common 
meatus.  Dorsally  this  space  is 
continuous  with  a  narrow  slit 
lying  between  the  superior  concha 
and  the  septum  and  called  the 
olfactory  cleft.  All'  these  pas- 
sages and  spaces  communicate 
more  or  less  directly  and  freely 
through  the  posterior  naris  or 
choana  with  the  pharynx. 

In  the  bones  about  the  nose  in  man  are  large  paired  air- 
spaces or  sinuses  that  communicate  with  the  exterior 
through  the  nasal  cavity.  These  spaces,  which  have  been 
very  fully  described  by  Schaeffer  (1916),  are  of  consid- 
erable size  and  are  lined  with  a  mucous  epithelium  con- 
tinuous with  that  of  the  nose.  They  are  somewhat  variable 
in  number  and  connections  and  yet  they  fall  more  or  less 

Fio.  2. — Diagram  of  a  trans- 
verse section  of  the  right  nasal 
cavity  in  man  made  at  the  plane 
indicated  by  the  vertical  dotted 
line  in  Fig.  1.  1,  inferior  concha; 
2,  middle  concha ;  3,  superior  con- 
cha; 4,  nasal  septum;  5,  inferior 
meatus;  6,  middle  meatus;  7, 
superior  meatus;  8,  common 
meatus;  9,  olfactory  cleft  (left 
side);  10,  ethmoid  cells;  11, 
maxillary  sinus. 


naturally  into  four  sets,  the  maxillary,  frontal,  and  sphe- 
noidal  sinuses  and  the  ethmoidal  cells.  Each  maxillary 
sinus  is  a  large  space  in  the  maxillary  bone  above  the 
teeth.  It  opens  by  a  considerable  slit  into  the  anterior  part 
of  the  middle  meatus.  (Figs.  1  and  2).  The  frontal  sinus,in 
the  frontal  bone  also  opens  into  the  middle  meatus  at  a 
point  above  and  anterior  to  the  opening  of  the  maxillary 
sinus.  Each  sphenoidal  sinus  opens  into  the  posterior  end 
of  the  appropriate  olfactory  cleft  in  a  region  known  as  the 
spheno-ethmoidal  recess.  The  remaining  accessory  nasal 
spaces,  the  ethmoid  cells,  are  more  or  less  variable ;  some 
of  them  open  into  the  middle  meatus  by  several  apertures 
well  above  the  slit  for  the  maxillary  sinus.  Others  open, 
more  commonly  by  a  single  aperture,  into  the  superior 
meatus.  In  addition  to  these  various  openings,  the  naso- 
lacrimal  duct,  by  which  the  lacrimal  secretions  from  the 
eye  are  carried  to  the  nasal  cavity,  opens  between  a 
pair  of  lips  on  the  lateral  wall  of  the  inferior  meatus  near 
its  anterior  extremity, 

2.  Nasal  Membranes.  The  nasal  vestibule  is  lined 
with  a  delicate  continuation  of  the  outer  skin.  The  walls 
of  the  deeper  part  of  the  nasal  cavity  are  covered  with  a 
mucous  membrane  which  is  divisible  into  two  regions,  the 
restricted  olfactory  region  in  the  dorsal  part  of  the  cavity 
and  the  much  more  extended  respiratory  region  embrac- 
ing the  remainder  of  the  cavity. 

The  mucous  membrane  of  the  respiratory  region  is 
reddish  in  color  and  consists  of  a  pseudo-stratified  epi- 
thelium containing  ciliated  cells  and  basal  cells  backed  up 
by  a  well  developed  tunica  propria.  (Fig.  3.)  The  cilia 
of  this  region  lash  towards  the  choana.  The  secretion 
covering  the  surface  of  the  epithelium  comes  from  numer- 


ous  branched  alveolo-tubular  glands  which  contain  both 
mucous  and  serous  cells. 

The  conchae  of  the  respiratory  region  have  long  been 
known  to  be  extremely  vascular  and  to  be  possessed  of  a 
structure  like  that  of  erectile  tissue.  This  is  especially 
true  of  their  edges.  They  can  be 
excited  through  reflex  channels  to 
considerable  enlargement  and  the 
swelling  thus  produced  may  be  suffi- 
cient to  close  completely  the  respir- 
atory passages.  It  is  believed  that 
this  high  vascularity  of  the  respira- 
tory region  is  concerned  with  the 
moistening  and  warming  of  the 
current  of  respiratory  air.  The 
secretions  of  this  portion  of  the  nose 
are  also  believed  to  be  inimical  to 
pathogenic  germs  and  thus  to  afford 
a  protection  to  the  deeper  parts  against  the  invasion 
of  disease. 

The  olfactory  region  in  man  is  yellowish  in  color  as 
it  is  in  the  calf  and  in  the  sheep.  In  the  dog  and  the 
rabbit  it  is  of  a  more  brownish  hue.  According  to  the 
older  anatomists  it  was  supposed  to  extend  in  man  over  the 
dorsal  half  or  even  more  of  the  nasal  cavity.  Von  Brunn 
( 1892 ) ,  however,  claimed  by  a  reconstruction  from  sections 
that  the  olfactory  epithelium  was  much  more  restricted 
than  had  been  originally  supposed.  According  to  this 
author  only  a  small  portion  of  the  superior  concha  and  a 
correspondingly  small  part  of  the  nasal  septum  represent 
the  unilateral  area  of  distribution  of  the  olfactory  nerve. 
This  area  in  one  subject  measured  257  sq.  mm.  and  in  an- 


Fio.  3. — Respiratory  epi- 
thelium from  the  nasal  cavity 
of  a  young  pig;  b,  basal  cell; 
c,  ciliated  cell.  After  Alcock, 
1910,  Fig.  2. 


other  238  sq.  mm.  The  more  recent  results  of  Bead 
(1908),  however,  show  that  in  man  the  olfactory  fibers 
spread  from  the  dorsal  portion  of  the  olfactory  cleft  ven- 
trally  over  the  superior  concha  almost  to  its  free  edge  and 
correspondingly  over  the  septum  to  about  one  third  its  ex- 
tent. (Fig.  4).  The  antero-posterior  spread  of  the  nerve, 

according  to  this  author,  is  about  twice 
that  of  its  ventral  distribution  on 
either  the  concha  or  the  septum; 
hence  the  whole  area  innervated  by 
each  olfactory  nerve,  if  spread  out 
flat,  would  be  approximately  square 
in  outline  and  not  far  from  25  mm. 
FIO.  4.  —olfactory  deft  to  a  side,  somewhat  over  twice  the 

of  man  opened  by   turning  MI  -AT  -r> 

the  nasai  septum  (s)  up-    extent  ascribed  to  it  by  von  Bruiin. 

ward;   the  blackened    area  o      /Mi?  TI     '  J.-L.    T  rrn  ii> 

shows  the  distribution  of        6.  Olfactory  iLpithelium.  The  olfac- 

the  olfactory  nerve.    After  " 

Figad3i  1908'   Plate    "'    tory  epithelium  has  been  an  object  of 

interest  to  histologists  for  a  long 
time.  As  early  as  1855  Eckhard  stated  that  in  the  frog 
it  was  composed  of  two  classes  of  elements,  long  epithelial 
cells  and  nucleated  fibers.  Which  of  these  served  as  the 
endings  of  the  olfactory  nerve  he  was  unable  to  say.  In 
the  same  year  Ecker  discovered  on  the  deep  face  of  the 
olfactory  epithelium  a  third  class  of  cells  subsequently 
called  by  Krause  (1876)  basal  cells.  (Fig.  5.)  These 
three  classes  of  elements  were  identified  in  a  number  of 
vertebrates  and  described  by  Schultze  (1856,  1862)  who 
expressed  the  belief  that  the  nucleated  fibers  were  sense 
cells  and  represented  the  true  endings  of  the  olfactory 
nerve  though  he  was  unable  to  demonstrate  a  connection 
between  these  cells  and  the  nerve. 

It  is  probable  that  the  connection  of  the  so-called 


olfactory  cell  with  the  olfactory  nerve-fiber  was  first  seen 
by  Babuchin  in  1872  who  showed  that  in  a  gold-chloride 
preparation,  fibers  could  be  traced  from  the  nerve  to  the 
cells  that  were  suspected  by  Schultze  to  be  sense  cells. 
In  1886   this  connection  was  demonstrated  with  much 
greater    certainty    in    methylen-blue    preparations    by 
Ehrlich  whose  results  were  con- 
firmed   the    following    year 
by  Arnstein. 

Because  of  the  transitoriness 
of  methylen-blue  preparations, 
the  results  of  Ehrlich  and  of 
Arnstein  were  looked  on  with 
some  suspicion  till  they  were 
reproduced  in  Golgi  preparations 
by  a  number  of  workers,  such  as 
Grassi  and  Castronovo  (1889) 
on  the  dog,  Ramon  y  Cajal  (1890) 
on  mammal  embryos,  Van 
Gehuchten  (1890)  on  rabbits,  von 
Brunn  (1892)  on  man,  Retzius 
(1892a,  1892b,  1894)  on  fishes, 
amphibians,  reptiles,  and  mam- 
mals, and  by  many  other  later 
workers  on  various  vertebrates. 
( Fig.  6 ) .  The  results  of  all  recent 
students  in  this  field  of  histology 
support  the  statement  that  the 

olfactory  epithelium  of  vertebrates  is  composed  of  at 
least  three  classes  of  cells :  basal  cells,  ordinary  epithelial 
or  substentacular  cells,  and  sense  cells  from  which  the 
olfactory  fibers  take  their  origin.  Thus  the  opinion  of 

Fio.  5. — Olfactory  epith- 
elium from  a  pig  embryo 
6H  inches  long;  b,  basal 
cell;  0,  olfactory  cell;  s,  sus- 
tentacular  cell.  After 
Alcock,  1910,  Fig.  10. 



Sclmltze  on  this  subject  has  been  unquestionably  and 
abundantly  confirmed. 

The  sustentacular  cells  are  the  chief  supporting  ele- 
ments of  the  olfactory  epithelium.  Each  of  these  cells 
has  a  distal  cylindrical  portion  that  contains  the  yellowish 
or  light  brownish  pigment  so  characteristic  of  the  olfac- 

Fio.  6. — Olfactory  epithelium  of  a  young  mouse  showing  the  olfactory  cells  and,  to  the  right, 
two  suatentacular  cells.    Golgi  preparation.     After  Retzius,  1892a,  Plate  10,  Fig.  2. 

tory  region.  The  nuclei  of  these  cells  are  oval  and  con- 
stitute the  outermost  zone  of  nuclei  in  the  epithelium. 
Their  proximal  portions  are  more  or  less  irregularly 
compressed  and  branched,  hence  the  outlines  of  these 
parts  are  commonly  jagged. 

The  basal  cells  form  a  single  row  of  block-like  elements 
on  the  proximal  face  of  the  olfactory  epithelium.  Their 
short  branching  processes  extend  distally  among  the  other 
cells  of  the  epithelium. 

The  olfactory  cells  are  the  most  numerous  of  the  three 
classes  of  cells  in  the  epithelium.  Their  nuclei  are  roundish 
with  well  marked  nucleoli  and  form  the  extensive  nucle- 
ated band  between  the  distal  zone  of  sustentacular  nuclei 



and  the  less  distinct  proximal  zone  of  basal  nuclei.  Each 
of  the  olfactory  nuclei  is  lodged  in  an  oval  cell-body. 
Proximally  this  tapers  rapidly  into  a  fine  olfactory  nerve- 
fiber  which  eventually  enters  the  olfactory  bulb  of  the 
brain.  Distally  the  body  of  the  cell  extends  as  a  somewhat 
coarser  rod-like  structure  to  the  outer  surface  of  the  olfac- 
tory epithelium  where  it  ter- 
minates in  a  small  enlargement. 
This  enlargement  has  been 
called  the  olfactory  vesicle  by 
Van  der  Stricht  (1909)  who 
ascribed  to  it  a  centrosomal 
origin  and  believed  it  to  play 
a  significant  part  in  olfactory 
reception.  The  olfactory  vesi- 
cle carries  a  cluster  of  proto- 
plasmic filaments,  the  olfactory 
hairs.  (Fig.  7).  These  hairs 
are  apparently  extremely  deli- 
cate and  are  easily  destroyed; 
hence  they  have  escaped  obser-j 
vation  by  many  workers.  They  were  probably  seen  in  the 
frog  as  early  as  1855  by  Eckhard,  but  they  were  first  gen- 
erally identified  and  thoroughly  studied  by  Schultze  (1856, 
1862)  in  a  number  of  vertebrates.  Apparently  they  are 
never  very  numerous;  Schultze  (1862)  found  that  in  the 
frog  there  were  five  to  six  hairs  on  each  olfactory  cell 
(Fig.  8),  and  von  Brunn  (1892)  and  Kallius  (1905)  re- 
corded six  to  eight  in  man.  Eetzius  (1894)  noted  two  to 
five  hairs  on  each  cell  in  the  snake  Tropidonotus.  Ballo- 
witz  (1904)  found  ten  to  twelve  or  more  in  Petromyzon, 
and  Alcock  (1910)  states  that  in  the  pig  the  number  varies 

Fia.  7. — Isolated  olfactory  cells 
and  sustentacular  cells  from  man. 
After  von  Brunn,  1892,  Plate  30, 
Fig.  4. 


from  five  to  eight,  Because  of  their  great  delicacy  the  ol- 
factory hairs  are  probably  seldom  observed  to  their  full 
length.  Schultze  (1856)  described  those  of  the  frog  as 
long,  but  Jagodowski  (1901)  has  shown 
that  in  the  pike  the  hair  may  be  twice  as 
long  as  the  olfactory  cell  itself,  (Fig.  9) 
and  may  reach  from  the  distal  end  of  the 
cell  through  the  whole  thickness  of  the 
superimposed  slime.  So  delicate  are  the 
distal  portions  of  these  hairs  that 
Jagodowski  has  proposed  for  them  the 
name  of  olfactory  flagella  or  lashes.  In 
the  opinion  of  this  author  the  so-called 
olfactory  hairs  are  only  the  proximal  ends 
of  these  lashes,  the  distal  part  having 
disappeared  in  the  course  of  preparation. 
The  lashes  can  be  demonstrated  by  means 
of  the  Golgi  method  or  by  osmic  acid. 
These  lashes  are  without  doubt  the  true 
receptive  elements  of  the  olfactory  cells. 
The  secretion  in  which  they  are  suspended 
and  whose  thickness  they  probably 
penetrate  is  produced  by  the  numerous 
olfactory  or  Bowman  glands  whose  ducts 
open  out  abundantly  through  the  olfac- 
tory epithelium. 

4.  Intermediate  Zone.  In  the  majority 
of  vertebrates  there  seems  to  be  a  fairly  sharp  boundary 
between  the  respiratory  epithelium  and  the  olfactory 
epithelium.  In  some  mammals,  however,  these  two  regions 
are  separated  by  a  considerable  intervening  area,  known 
as  the  intermediate  zone.  This  was  first  described  by 

Fio.  8. — Isolated 
olfactory  cell  and 
suatentacular  cell 
from  a  frog.  After 
Schultze,  1862, 
Plate  1,  Fig.  4. 


Grassi  and  Castronovo  (1889)  in  the  dog,  and  subsequently 

was  identified  by  Alcock  (1910)  in  the  pig.  In  this  mammal 

the  epithelium  of  the  intermediate  zone  is  thicker  than  that 

of  the  respiratory  region  and  thinner  than  that  of  the 

olfactory  region.     Besides  basal  cells  it  possesses  two 

types  of  epithelial  cells,  ciliated  cells  like 

those  of  the  respiratory  epithelium  and  1 

non-ciliated  sustentacular  cells  like  those 

of  the  olfactory  region.    It  also  contains 

many  olfactory  cells,  but  these  cells  are 

not  as  numerous  in  the  intermediate  zone 

as    they    are    in    the    olfactory    region 

where  they  are   said   to  make  up   about 

seventy  per  cent  of  the  cells  present.    It 

is    plain    from    the    accounts    given    that 

the    intermediate    zone    is    a    region    of 

transition   between   the    two    chief   nasal       FIQ.Q— 

preparation    of   an 

regions,  the  olfactory  and  the  respiratory.    alfpikery(Eesox)roin 
5.  Polymorphic  Cells.     In  most  verte-    Srb.3?  ffl 

brates   the    olfactory   cells    exhibit   great    process  nearl" 

.  ,,  .  .  ,,  -.-          .,  „    ,  shown,  but  also  the 

umtormity   01    structure.     In   the   fishes,    iong  peripheral  oi- 

factory     flagellum. 

however,  Dogiel  (1887)  has  called  attention 
to  a  polymorphism  among  these  elements, 
and  he  has  described  in  addition  to  the  ordinary  type  of 
spindle-shaped  olfactory  cell,  cylindrical  olfactory 
cells  and  conical  olfactory  cells.  These  three  types 
have  been  identified  by  Morrill  (1898)  and  by  Asai 
(1913)  in  a  selachian  (Mustelus)  and  by  Jagodowski 
(1901)  in  the  pike  (Esox).  To  what  extent  this 
polymorphism  occurs  in  other  vertebrates  and  how 
important  it  is  for  a  right  understanding  of  the  action 
of  the  olfactory  organ  has  not  yet  been  determined. 



6.  Sense  Buds.     In  1884  Blaue  described  what  he  be- 
lieved to  be  sense  buds  in  the  olfactory  epithelium  of 
certain  fishes  and  amphibians.     This  observation  was  not 
confirmed  by  later  workers  and  it  appears,  as  Betzius 
(1892b)  has  remarked,  that  the  so-called  sense  buds  are 
not  true  buds  but  folds  or  bands  of  olfactory  epithelium 
seen  in  transverse  section.     The  buds  subsequently  de- 
scribed by  Disse  (1896b)  in  the  nose  of  the  calf  and  shown 
by  him  to  be  supplied  by  free-nerve  terminations  are  be- 
lieved by  this  author  to  be  concerned  with  taste  rather 
than  with  smell.     These  structures,  however,  are  claimed 
by   Kamon    (1904)    not  to   be   true   buds  but  bud-like 
appearances  produced  by  the  mouths  of  the  Bowman 
glands.      If  this  is  so,  no  sense  buds  of  any  kind  are 
known  in  the  olfactory  epithelium  of  vertebrates. 

7.  Free-nerve  Endings.  In  1889  Grassi  and  Castronovo 
with  some  uncertainty  described  from  the   epithelium 
of  the  intermediate  zone  of  the  dog  what  they  regarded  as 
free-nerve  endings.    Whether  these  were  end-organs  of 
the  olfactory  nerve-fibers  or  not,  they  were  unable  to 
determine.     In  1892  similar  endings  were  observed  by  von 
Brunn  at  the  border  of  the  respiratory  region  in  man. 
Von  Brunn  believed  these  endings  to  be  terminals  of  the 
trigeminal  nerve  and,  apparently  by  mistake,  mentioned 
Ramon  y  Cajal  as  their  discoverer.     Free-nerve  endings 
in  the  olfactory  region  were  subsequently  recorded  by 
Retzius  (1892b)  in  the  mouse  and  frog,  by  von  Lenhossek 
(1892)  in  the  rabbit,  by  Morrill  (1898)  in  Mustelus,  by 
Jagodowski   (1901)   in  Esox,  by  Kallius   (1905)   in  the 
calf,  and  by  Read  (1908)  in  the  kitten.     Morrill 's  obser- 
vation for  Mustelus  has  recently  been  confirmed  by  Asai 
(1913).     Hence  there  seems  to  be  no  doubt  that  in  addition 


to  the  olfactory  cells,  free-nerve  endings  occur  in  the  ol- 
factory epithelium  of  vertebrates. 

The  source  of  the  nerve-fibers  from  which  the  free- 
endings  of  the  olfactory  epithelium  arise  is  not  definitely 
settled.  The  fact  that  these  endings  may  be  very  near  the 
outer  surface  of  the  olfactory  epithelium  shows  that  they 
are  not  due  to  the  incomplete  impregnation  of  fibers  from 
the  olfactory  cells  as  was  suggested  by  Van  Gehuchten 
(1890).  Free-endings  like  those  in  the  olfactory  region 
also  occur  in  the  respiratory  region  and  here  the  only  pos- 
sible source  for  them  is  the  trigeminal  nerve;  hence  it  is 
probable  that  this  nerve  is  also  the  source  of  the  free- 
nerve  endings  of  the  olfactory  region.  This  opinion  is  sup- 
ported by  the  observations  of  Rubaschkin  (1903)  who  has 
shown  that  in  certain  portions  of  the  olfactory  epithelium 
of  the  developing  chick  the  two  sets  of  fibers,  those  from 
the  olfactory  nerve  and  those  from  the  trigeminal  nerve, 
take  somewhat  different  courses  and  that  the  trigeminal 
fibers  are  the  fibers  that  give  rise  to  the  free-endings. 
(Fig.  10).  Thus  such  evidence  as  there  is  favors  the 
opinion  first  expressed  by  von  Brunn  and  subsequently 
reiterated  by  a  number  of  investigators,  that  the  free- 
nerve  endings  of  the  olfactory  region  are  from  the  tri- 
geminal fibers.  The  vertebrate  olfactory  epithelium, 
therefore,  has  two  types  of  nerve  terminations,  olfactory 
cells  as  the  exclusive  receptors  for  the  olfactory  nerve 
and  free-nerve  endings  as  the  probably  exclusive  endings 
for  the  trigeminal  nerve. 

8.  Development  of  Olfactory  Nerve.  Since  the  fibers 
from  the  olfactory  cells  pass  as  olfactory  nerve-fibers 
to  the  olfactory  bulb  and  terminate  there  without  direct 
connections  with  any  other  cells,  the  olfactory  cells  in  the 



nasal  epithelium  must  be  their  cells  of  origin,  as  in  fact 
was  shown  to  be  the  case  for  the  chick  by  Disse  (1896a, 
1897).  Here  the  olfactory  nerve-fibers  have  been  demon- 
strated to  grow  from  certain  olfactory  epithelial  cells  into 
the  olfactory  bulb,  the  epithelial  cells  acting  in  all  respects 
like  neuroblasts  Bedford  (1904).  The  trigeminal  fibers 

Fio.  10. — Olfactory  epithelium  of  an  embryo  chick  (ninth  day)  showing  olfactory  cells, 
sustentacular  cells,  and  free-nerve  endings  of  fibers  from  ganglion  cells  of  the  trigeminal 
nerve.  After  Rubaschkin,  1903,  Fig.  3. 

must  on  the  other  hand  grow  from  trigeminal  ganglion 
cells  into  the  olfactory  epithelium  there  to  terminate  as 
free-nerve  endings,  but  of  this  there  is  at  present  no  di- 
rect evidence. 

9.  Comparative  Anatomy  of  Olfactory  Organs.  The 
nasal  organs  in  the  lower  vertebrates  are  very  different 
from  those  in  man.  In  Amphioxus  a  single  sensory  pit 
slightly  to  the  left  of  the  median  dorsal  line  of  the  head 
and  connected  with  the  anterior  end  of  the  nerve-tube  is 
assumed  to  be  an  olfactory  organ.  If  this  is  so,  it  is  prob- 
able that  this  pit  corresponds  to  the  single  median  olfac- 


tory  sac  in  the  cyclostomes  notwithstanding  the  fact  that 
this  sac  shows  evidence  in  its  deeper  parts  of  being  a 
double  organ.  In  consequence  of  single  nasal  openings 
Amphioxus  and  the  cyclostomes  are  commonly  contrasted 
with  other  fishes,  and  in  fact  with  all  other  vertebrates, 
and  are  called  monorhine.  Those  in  which  the  olfactory 
organs  are  obviously  paired  have  been  designated  as 

In  the  sharks  and  rays  the  paired  olfactory  pits  are 
situated  usually  on  the  ventral  side  of  the  snout.  (Fig.  11) . 
The  single  opening  of  each  pit  is  more  or  less  divided  by  a 
fold  of  skin  into  an  anterior  inlet  and  a  posterior  outlet 
the  latter  sometimes  leading  into  the  mouth.  As  the  fish 
swims  through  the  water  and  particularly  as  it  takes 
water  into  its  mouth  in  breathing,  a  current  of  water  is 
passed  through  eadi  of  its  olfactory  sacs.  In  this  way  the 
olfactory  organs  become  associated  with  the  respiratory 
current,  a  condition  that  is  more  pronounced  in  the  lung- 
fishes  than  in  the  sharks  and  rays,  for  in  the  lung-fishes 
the  anterior  apertures  are  external  and  form  true  anterior 
nares,  and  the  posterior  openings  lie  within  the  mouth 
and  correspond  to  the  choanas  of  higher  vertebrates.  In 
the  highly  specialized  bony  fishes,  the  paired  olfactory  pits 
are  almost  always  on  the  dorsal  aspect  of  the  head  and 
quite  distant  from  the  mouth.  Each  pit  has  two  entirely 
separate  openings,  an  anterior  inlet  and  a  posterior  outlet. 
By  means  of  these  two  openings  a  current  of  water  enters 
and  leaves  each  pit.  This  current  is  produced  either  by 
ciliary  action  within  the  pit  ( Amiurus)  or  by  the  action  of 
the  muscles  associated  with  the  jaws  and  gills  (Fundulus). 
In  bony  fishes,  then,  the  olfactory  pits  are  purely  recep- 
tive and  are  in  no  direct  way  connected  with  the  respira- 



tory  current  as  they  are  in  the  sharks  and  rays,  and  in 
the  lung-fishes. 

In  the  air-inhabiting  vertebrates  each  olfactory  sac 
possesses,  as  in  man,  an  external  inlet,  the  anterior  naris, 
and  a  posterior  outlet,  the  choana,  opening  into  the  mouth 
or  the  pharynx.  The  olfactory  sacs  are  relatively  simple 
in  amphibians,  but  become  progressively  more  compli- 

Fia.  11. — Ventral  view  of  the  head  of  a  shark  (Scyllium)  showing  the  olfactory  pita  in  rela- 
tion to  the  mouth. 

cated  in  reptiles  and  birds,  and  vastly  more  so  in  mam- 
mals. Here  the  surface  of  the  sac  is  enormously  extended 
through  the  development  of  lateral  folds  or  conchae  which 
may  be  further  complicated  by  the  production  of  second- 
ary folds.  In  mammals  the  more  ventral  of  these  conchaa, 
those  attached  to  the  maxillary  bone,  are  apparently  not 
concerned  with  olfaction,  but  lie  in  the  purely  respiratory 
region  of  the  nasal  chamber.  The  more  dorsal  conchae 
those  from  the  ethmoid  bone,  serve  as  olfactory  surfaces. 
It  has  been  shown  that  in  some  mammals,  as  for  instance 
in  Orycteropus,  there  may  be  upwards  of  ten  olfactory 


conchae.  Forms  that  possess  these  larger  numbers  of 
conchas  are  known  to  be  keen-scented  and  are  termed  mac- 
ro smatic.  Those  in  which  the  number  of  olfactory  conchae 
is  small,  four  or  fewer,  such  as  the  seals,  some  whales, 
monkeys,  and  man,  are  known  to  be  less  acute  of  smell 
and  are  called  microsmatic.  Others  again,  such  as  the 
toothed  whales,  porpoises  and  the  like,  in  which  the  olfac- 
tory organ  has  almost  completely  degenerated,  are  sup- 
posed to  be  devoid  of  olfaction  and  are  called  anosmatic. 
In  such  forms  the  nasal  cavities  have  lost  their  original 
sensory  function  and  have  come  to  be  of  importance  only 
in  connection  with  respiration,  a  purely  secondary  relation. 


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Contents. — 1.  Nerves  of  Olfaction.  2.  Passage  of  Air 
through  the  Nasal  Cavity.  3.  Minimum  Stimulus. 
4.  Physical  Condition  of  Stimulus,  Gas  or  Solution?  5. 
Olfaction  in  Fishes.  6.  Fatigue  and  Exhaustion.  7.  Quali- 
ties of  Odors.  8.  Chemical  Relations  of  Odors.  9.  Inade- 
quate and  Adequate  Stimuli.  10.  Olfactory  Reflexes. 
11.  Bibliography. 

1.  NERVES  of  Olfaction.  The  olfactory  region  of  verte- 
brates has  been  shown  to  possess  olfactory  cells  as  ter- 
minations of  the  olfactory  nerve  and  free-nerve  endings 
representing  in  all  probability  the  trigeminal  nerve.  It 
has  long  been  the  opinion  of  investigators  that  the  olfac- 
tory sense  is  mediated  by  the  endings  of  the  olfactory 
nerve,  but  this  opinion  has  not  been  without  its  opponents. 
Thus  Magendie,  in  a  series  of  publications  beginning  in 
1824,  came  to  the  conclusion  that  the  trigeminal  nerve  was 
the  nerve  of  olfaction  and  that  the  so-called  olfactory 
nerve  was  one  whose  function  was  wholly  unknown.  His 
opinion  was  based  in  part  upon  experiments  on  the  dog. 
After  the  olfactory  nerves  of  this  animal  had  been  cut,  it 
was  found  still  to  respond  to  acetic  ether  and  to  ammonia. 
Even  when  blindfolded  a  dog  with  severed  olfactory 
nerves  would  seize  cheese  or  meat  but  it  would  not  eat 
meat  sprinkled  with  tobacco.  It  was  pointed  out  by 
Magendie 's  critics  that  many  of  the  stimulating  sub- 
stances used  by  him,  such  as  ammonia  and  the  like,  not 



only  possessed  odor  but  were  irritants  for  mucous  sur- 
faces generally  and  thus  without  reference  to  olfaction 
could  call  forth  vigorous  responses.  Magendie,  however, 
claimed  that  his  results  were  not  dependent  upon  these 
substances,  but  could  be  demonstrated  by  the  use  of  non- 
irritants,  such  as  lavender  oil. 

Magendie 's  opinion  that  the  trigeminal  nerve  was  the 
nerve  of  olfaction  was  opposed  almost  from  the  beginning. 
Eschricht  in  1825  pointed  to  numerous  cases  of  persons 
who  were  anosmic  in  consequence  of  the  absence  of  the 
olfactory  nerve  or  of  its  degeneration.  Bishop  in  1833 
described  a  case  of  paralysis  of  the  trigeminal  nerve  in 
which  there  was,  however,  full  retention  of  olfaction. 
Picht  (1829)  and  Duges  (1838),  both  of  whom  were  incap- 
able of  olfaction  in  the  ordinary  sense  of  the  word,  were 
nevertheless  easily  stimulated  through  their  nasal  mem- 
branes by  the  vapor  of  acetic  ether,  or  of  ammonia.  Val- 
entin (1839)  found  that  a  normal  rabbit  would  sniff  the 
body  of  a  dead  one,  but  that  a  rabbit  whose  olfactory 
nerves  had  been  cut  would  not  thus  respond.  Schiff 
(1859)  experimented  on  five  pups,  in  four  of  which  the 
olfactories  were  severed,  the  fifth  being  retained  in  a  nor- 
mal condition  as  a  control.  After  recovery  from  the  op- 
eration, the  four  pups  in  which  the  nerves  had  been  cut 
were  unable  to  find  the  mother's  nipples,  and  did  not  dis- 
tinguish between  a  man  and  the  mother  though  they 
turned  their  heads  away  and  sneezed  when  ammonia  or 
ether  was  administered.  Acetic  acid  stimulated  them  only 
when  its  vapor  was  very  concentrated.  These  and  many 
other  similar  results  completely  overthrew  Magendie 's 
contention  and  showed  that,  though  the  trigeminal 
endings  were  concerned  with  the  reception  of  what  may  be 


called  irritants,  true  olfaction  was  accomplished  only 
through  the  olfactory  terminals,  which  have  to  do  with 
delicate  perfumes,  aromas,  and  the  like,  many  of  which 
were  associated  with  food. 

The  recognition  in  nasal  stimulation  of  the  two  classes 
of  substances,  irritants  acting  on  trigeminal  terminals, 
and  true  odors  affecting  the  olfactory  endings,  is  of  funda- 
mental importance,  and  the  failure  to  appreciate  this 
distinction  is  responsible  in  part  at  least  for  much  of  the 
confusion  that  exists  in  what  has  been  written  on  the 
olfactory  stimulus.  As  early  as  1851  Frohlich  pointed  out 
this  distinction  and  called  attention  to  the  fact  that  irri- 
tants or  stimuli  for  the  fifth  nerve  ordinarily  induce 
vigorous  reflexes,  respiratory  and  the  like,  whereas  true 
odors  are  in  nature  much  milder  and  seldom  call  forth 
strong  responses.  It  is  quite  possible  that  some  materials 
are  stimuli  for  both  classes  of  end-organs;  thus  tobacco 
smoke  not  only  carries  with  it  an  aroma  or  true  odor  but 
also  acts  as  an  irritant,  These  two  actions,  however,  may 
depend  upon  different  chemical  substances  in  the  smoke. 
Other  stimuli  such  as  oil  of  mustard  or  possibly  ammonia, 
that  are  chemically  much  more  homogeneous  than  tobacco 
smoke,  may  affect,  nevertheless,  both  sets  of  receptors  and 
thus  exhibit  the  characteristics  of  both  irritants  and 
true  odors.  A  revision  of  the  so-called  olfactory  stimuli 
from  this  standpoint  is  much  to  be  desired. 

2.  Passage  of  Air  through  the  Nasal  Cavity.  In  ordi- 
nary respiration  in  man  the  passage  of  air  through  the 
nasal  cavity  does  not  necessarily  excite  olfaction  at  once. 
Sooner  or  later,  however  the  odor  may  be  slightly  sensed 
after  which  a  few  deep  breaths  or  sniffing  movements 
are  usually  made,  whereupon  full  stimulation  ensues. 


The  course  that  the  current  of  air  takes  through  the 
nasal  chamber  during  quiet  respiration  has  been  studied  in 
several  ways.  Paulsen  in  1882  published  the  results  of  ex- 
periments 011  the  human  cadaver.  He  opened  the  nasal 
cavity  by  sawing  through  the  head  of  a  cadaver  close  to 
the  median  plane.  Pieces  of  red  litmus-paper  were  then 
placed  on  different  parts  of  the  nasal  surface  and  the  two 
halves  of  the  head  were  brought  together  again.  By 
means  of  a  bellows  attached  to  the  trachea  of  the  cadaver, 
the  current  of  air  that  in  life  passes  through  the  nasal 
chambers  was  imitated.  This  artificial  current  was 
charged  with  ammonia  and  thus  a  means  was  given  of  in- 
dicating the  spread  of  the  current  by  the  location  of  the 
pieces  of  litmus-paper  that  changed  from  red  to  blue.  As 
a  result  of  this  test  it  was  found  that  the  inspired  air  took 
a  curved  course  from  the  naris  to  the  choana.  (Fig.  12). 
The  highest  part  of  this  curve  was  near  the  middle  of  the 
nasal  cavity,  but  this  never  reached  a  point  as  high 
as  the  olfactory  cleft.  When  the  current  was  reversed 
by  causing  it  to  enter  at  the  choana  and  emerge  at 
the  external  naris,  as  in  expiration,  the  direction  of  the 
current  was  found  to  be  much  the  same  as  in  inspiration 
except  that  a  somewhat  lower  course  was  followed.  Thus 
in  both  inspiration  and  expiration  the  current  of  air  is 
limited  to  what  is  more  generally  regarded  as  the  respira- 
tory region  of  the  nasal  cavity,  the  olfactory  region  being 
essentially  undisturbed. 

Paulsen 's  results  were  confirmed  in  all  essential  par- 
ticulars by  a  number  of  later  investigators  including 
Franke,  Zwaardemaker,  Danziger,  and  Rethi,  who  worked 
on  dead  animals  and  human  cadavers  by  methods  not  un- 
like those  used  by  Paulsen.  Franke  (1893)  sawed  open 


the  head  of  a  human  cadaver  in  the  median  plane,  replaced 
the  nasal  septum  with  glass  and  by  means  of  an  artificially 
produced  respiratoiy  current  showed  that  smoke  in  its 
passage  through  the  nasal  cavity  remained  in  the  so-called 
respiratory  region.  He  observed,  however,  that  both  in- 
spiration and  expiration  were  accompanied  by  strong 

Fio.  12. — Diagram  of  the  right  nasal  cavity  of  man  laid  open  and  showing  by  arrows  the 
direction  of  the  inspired  current  of  air  over  the  nasal  septum  (right  half  of  figure)  and  over 
the  lateral  wall  (left  half  of  figure).  After  Paulsen,  1882. 

eddies  in  the  moving  air.  Kayser  (1890)  aspirated  very 
light  magnesia  powder  into  the  respiratory  current  of  a 
quietly  breathing  normal  subject  and  then  inspected  the 
nasal  surfaces  by  means  of  a  rhinoscope.  The  magnesia 
particles  accumulated  on  the  moist  surfaces  of  the  respir- 
atory portions  of  the  nose  and  not  on  those  of  the  olfac- 
tory region,  thus  confirming  Paulsen 's  results  but  by  a 
method  that  was  by  no  means  so  artificial  as  that  employed 
by  other  workers.  It  may,  therefore,  'be  regarded  as 


fairly  well  established  that  the  current  of  air  that  sweeps 
through  the  nasal  cavity  in  quiet  respiration  is  limited 
chiefly  to  the  non-olfactory  portion  of  that  cavity.  Ac- 
cording to  Paulsen  and  to  Zwaardemaker  this  current 
even  in  its  eddying  effect  does  not  rise  above  the  lower 
edge  of  the  middle  concha  or  at  most,  according  to 
Franke,  the  lower  edge  of  the  superior  concha.  This 
limitation  is  probably  more  pronounced  in  expiration 
than  in  inspiration. 

Although  the  experimental  evidence  does  not  show  that 
the  respiratory  current  spreads  to  the  olfactory  surface 
of  the  nose,  odorous  particles  must  in  some  way  reach  this 
situation.  Zwaardemaker  (1895)  was  led  to  believe  that 
the  diffusion  of  these  particles  played  an  important  part 
in  this  process,  but  diffusion  is  a  relatively  slow  operation 
and  it  is  very  doubtful  if  it  is  a  significant  factor  in  carry- 
ing the  odorous  material  to  the  olfactory  receptor.  It 
seems  more  probable  that  the  shifting  pressures  that 
accompany  respiration  and  the  slight  eddies  that  are 
formed  in  the  general  current  are  responsible  for  a  grad- 
ual change  of  air  in  the  olfactory  cleft.  The  change  thus 
produced  is  probably  too  slight  to  be  detected  easily  by 
the  means  heretofore  employed  in  tracing  the  current  and 
yet  it  may  be  sufficient  to  initiate  such  olfaction  as  occurs 
in  quiet  respiration.  Olfaction  thus  once  begun  would 
naturally  excite  sniffing  and  this  process  seems  to  be 
entirely  sufficient  to  account  for  a  rapid  change  of  air  in 
the  olfactory  cleft  whereby  olfaction  would  be  brought  to 
full  height.  Thus  air  currents  are  certainly  the  chief  if 
not  the  sole  factors  concerned  with  transporting  the 
odorous  particles  to  the  olfactory  membranes. 

The  accumulation  of  odorous  materials  on  the  olfac- 


tory  surfaces  may  be  much  intensified  by  the  condensa- 
tion of  moisture  within  the  nasal  cavity.  Zwaardemaker 
(1917)  has  called  attention  to  the  fact  that  a  fog  formed 
from  a  vaporized  salt  solution  is  very  much  less  stable 
when  it  includes  odorous  substances  than  when  it  does 
not.  This  condition  is  believed  to  depend  upon  the  elec- 
tric charges  earned  by  the  particles  concerned,  and 
Durand  (1918a,  1918b)  recently  claimed  that  olf action 
is  more  or  less  dependent  upon  an  appropriate  hygro- 
metric  state  in  the  olfactory  atmosphere  and  that  what- 
ever facilitates  the  condensation  of  watery  vapor  there 
facilitates  olfaction. 

Among  the  older  physiologists  Bidder  (1844)  main- 
tained that  olfaction  was  possible  on  inspiration  and  that 
expired  air  could  not  stimulate  the  organ  of  smell.  Paul- 
sen's  observations  show  that  this  opinion  is  improbable 
and  the  direct  test  of  breathing  odorous  air  in  through 
the  mouth  and  out  through  the  nasal  cavity  has  de- 
monstrated that  it  is  quite  erroneous.  The  olfactory 
sensations  produced  on  expiration  are  noticea.bly  less 
than  on  inspiration  and  this  is  probably  due  partly  to  the 
lower  course  maintained  in  the  nasal  cavity  by  the  ex- 
pired air  and  partly  to  the  previous  elimination  of  much 
of  the  odorous  material  by  attachment  to  the  moist  sur- 
faces of  the  mouth,  pharynx,  and  other  parts  over  which 
the  air  passes  on  its  way  to  the  nasal  chamber.  Never- 
theless, as  Nagel  (1904)  has  pointed  out,  the  odors  of  our 
food  during  mastication  are  the  results  of  stimulating 
material  that  reaches  the  olfactory  surfaces  through  the 
choanaB  rather  than  through  the  external  nares.  The 
importance  of  these  odors  in  promoting  the  various  kinds 


of  digestive  reflexes,  muscular,  secretory,  and  so  forth, 
has  long  been  recognized. 

3.  Minimum  Stimulus.  The  common  belief  that  the 
olfactory  stimulus  consists  of  minute  material  particles 
suspended  in  the  air  current  of  the  olfactory  organ  is 
supported  by  the  observation  that  odors  may  be  carried 
on  the  wind  in  a  definite  direction  many  miles.  Odors 
do  not  emanate  from  a  given  center  and  disperse  in  all 
directions  as  sound  and  light  do.  Moreover  many  sub- 
stances, such  as  arsenic,  that  are  odorless  under  ordinary 
circumstances,  give  out  an  odor  after  they  have  been 
heated  sufficiently  to  volatilize.  The  fact,  discovered  in 
1917  by  Woodrow  and  Karpman,  that  the  adaptation  time 
for  olfaction — the  time  needed  for  an  olfactory  sensation 
to  wane  completely — is  directly  proportional  to  the  vapor 
tension  of  the  odorous  material  shows  that  olfactory 
stimulation  is  due  to  the  activity  of  gaseous  particles. 
These  and  other  like  observations  have  led  to  the  conclu- 
sion, now  generally  accepted,  that  the  olfactory  organs 
are  normally  stimulated  by  material  particles,  and  not 
by  disturbances  of  a  non-material  character. 

Some  odorous  bodies  such  as  musk  are  well  known 
to  give  out  these  material  particles  for  a  very  considerable 
time  without  appreciably  changing  weight.  From  the 
standpoint  of  the  receptor  this  indicates  that  olfaction 
is  called  forth  by  an  infinite simally  small  amount  of  sub- 
stance, and  measurements  directed  toward  testing  this 
question  justify  the  conclusion.  These  measurements 
have  been  made  in  a  variety  of  ways. 

One  method  of  procedure  is  that  of  evaporating  a 
given  weight  of  odorous  material  in  a  known  volume  of  air 
and  then  testing  the  air  by  sniffing  it.  This  method  lends 


itself  readily  to  the  determination  of  absolute  measure- 
ments but  it  is  not  so  easily  applied  to  questions  involv- 
ing the  comparison  of  odors.  For  the  measurement  of 
olfactory  acuity,  but  especially  for  the  comparison  of 
odors,  Zwaardemaker  invented  an  ingenious  piece  of  ap- 
paratus called  an  olfactometer.  (Fig.  13).  This  consists 
of  two  tubes  that  slide  one  within  the  other  and  so  shaped 
that  one  end  of  the  inner  tube  may  be  applied  to  the 
nostril.  The  odorous  material  is  carried  on  the  inner 
surface  of  the  outer  tube.  When  the  inner  tube,  which 
is  graduated,  is  slipped  into  the  outer  one  so  as  to  cover 

—i — i — i — i — i — i — i — i — r 

a     8765*321 


FIG.  13. — Simple  rubber  olfactometer.    After  Zwaardemaker,  1895,  Fig.  14. 

completely  its  inner  face  and  air  is  drawn,  into  the  nostril 
through  the  tube,  the  odorous  surface  being  covered  gives 
out  no  particles  and  no  odor  is  perceived.  If,  now,  the 
inner  tube  is  withdrawn  a  certain  distance  so  that  a 
given  surface  of  odorous  material  is  exposed  to  the  cur- 
rent of  air,  odorous  particles  escape  into  the  current  and 
these  may  be  sufficient  in  amount  to  call  forth  olfaction. 
By  adjusting  the  inner  tube  in  relation  to  the  outer  one 
whereby  more  or  less  of  the  odorous  surface  is  exposed, 
a  point  can  be  found  where  minimum  stimulation  occurs. 
The  amount  of  odorous  substance  delivered  under  these 
circumstances  to  the  air  current  has  been  designated  by 
Zwaardemaker  as  an  olfactie,  the  unit  of  olfactory  stimu- 
lation. Having  determined  for  a  given  substance  the 
area  necessary  for  the  delivery  of  one  olfactie,  doubling 



that  surface  by  an  appropriate  movement  of  the  inner 
tube  will  produce  a  stimulus  of  two  olfacties  and  so 
forth.  Thus  a  graded  series  of  measured  olfactory  stim- 
uli can  easily  be  obtained.  Further,  by  using  outer 
tubes  carrying  different  odorous  substances,  various  com- 
parisons can  be  instituted  as  measured  in  olfacties. 
Moreover,  a  double  olfactometer  (Fig.  14)  may  be  easily 

14. — Double  olfaotometer.     After  Zwaardemaker,  1895,  Fig.  15. 

devised  in  that  two  single  olf  actometers  may  be  combined 
so  that  one  current  carrying  an  odorous  material  of  a 
given  concentration  may  be  introduced  into  one  nostril 
and  another  carrying  a  second  odorous  substance  of 
known  concentration  can  be  introduced  into  the  other 
nostril,  or  both  currents  may  be  united  and  the  odorous 
mixture  thus  produced  can  be  let  into  one  nostril.  Thus 
a  variety  of  comparisons  may  be  easily  made. 

Van  Dam  (1917b)  has  modified  Zwaardemaker 's  ap- 
paratus by  applying  the  odorous  material  in  the  form  of 


a  rod  instead  of  a  coating  to  the  inside  of  a  tube.  The 
rod  is  made  of  paraffin  mixed  in  a  definite  proportion  with 
the  odorous  substance  and  the  extent  to  which  the  rod  is 
exposed  in  the  olfactometer  tube  is  a  measure  of  the 
concentration  of  the  odorous  particles  in  the  air  current. 
Rods  of  metal,  platinum,  gold,  or  zinc,  have  also  been 
used ;  these  have  been  charged  by  immersing  them  in  an 
atmosphere  of  odorous  material  for  a  given  length  of 
time  and  then  tested.  The  odorous  particles  gather  on 
their  surfaces  and  are  subsequently  freed.  The  success  of 
this  method  makes  it  clear  tha.t  in  the  original  evapora- 
tion method  more  or  less  of  the  odorous  material  must 
become  ineffective  in  that  it  adheres  to  the  walls  of  the 
container  in  which  the  evaporation  is  carried  out. 

As  a  means  of  avoiding  these  and  other  difficulties 
Allison  and  Katz  (1919)  have  recently  employed  in  the 
testing  of  stenches  a  type  of  odiometer  that  for  accuracy 
of  work  bids  fair  to  replace  most  of  the  other  devices. 
It  consists  of  a  number  of  Venturi-type  flow-meters  so 
arranged  that  a  measured  volume  of  air  can  be  passed 
at  a  uniform  rate  through  or  over  the  chemical,  and  this 
air  is  then  mixed  with  another  measured  volume  of  pure 
air  also  flowing  at  a  uniform  rate.  The  concentration  of 
the  chemical  is  measured  by  determining  its  loss  in 
weight  after  a  measured  volume  of  air  has  passed 
through  or  over  it.  From  this  loss  of  weight  and  the  to- 
tal volume  of  air  with  which  the  chemical  has  been  mixed, 
the  concentration  in  milligrams  per  liter  of  air  is  deter- 
mined. The  mixture  of  air  and  chemical  passes  finally 
through  a  tube  with  a  glass  funnel  at  the  open  end.  The 
funnel  is  placed  over  the  nose  of  the  person  who  by  a  sin- 
gle inhalation  tests  the  mixture.  The  odors  are  rated  ar- 
bitrarily as  detectable,  faint,  quite  noticeable,  strong  or 


very  strong.  By  this  means  extremely  accurate  quanti- 
tative results  can  be  obtained. 

In  testing  olfactory  acuity  the  majority  of  workers 
have  used  the  method  of  evaporating  a  known  weight  of 
substance  in  a  given  volume  of  air.  By  this  method  Val- 
entin (1848)  found  that  1/2,000,000  of  a  milligram  of  oil 
of  rose  per  cubic  centimeter  of  air  was  odorous.  Assum- 
ing that  100  cubic  centimeters  of  this  mixture  were  nec- 
essary for  olf action,  he  concluded  that  the  total  weight  of 
oil  of  rose  used  in  this  operation  was  the  very  small 
amount  of  1/20,000  of  a  milligram.  Valentin  also  found 
that  water  containing  1/2,000,000,000  of  its  weight  of 
tincture  of  musk  had  a  perceptible  odor  whereas  water 
containing  only  1/3,300,000,000  of  this  tincture  could  not 
be  distinguished  from  ordinary  water.  One  gram  of  the 
odorous  mixture  called  forth  the  characteristic  smell  and 
contained  only  1/2,000,000  of  a  milligram  of  tincture 
of  musk. 

More  significant  measurements  were  made  by  Fischer 
and  Penzoldt  (1886)  on  chlorphenol  and  mercaptan.  One 
milligram  of  chlorphenol  was  evaporated  in  a  room  of 
230  cubic  meters  capacity  and  was  thoroughly  mixed  with 
the  air.  This  dilution  called  forth  an  unquestionable  ol- 
factory sensation.  It  contained  1/230,000,000  of  a  milli- 
gram of  chlorphenol  per  cubic  centimeter  of  air  or,  if  it 
is  assumed  that  50  cubic  centimeters  of  air  are  the  mini- 
mum needful  for  olfaction,  the  total  amount  of  chlor- 
phenol necessary  was  found  to  be  1/4,600,000  of  a 
milligram.  By  a  similar  method  it  was  shown  that 
mercaptan,  a  liquid  with  a  penetrating  garlic  odor,  could 
be  recognized  at  a  concentration  of  1/23,000,000,000  of  a 
milligram  per  cubic  centimeter,  a  concentration  that 
would  yield  1/460,000,000  of  a  milligram  for  every  50 


cubic  centimeters  of  air.  Notwithstanding  this  infinites- 
imally  small  amount  of  mercaptan,  the  quantity,  just 
designated  was  estimated  by  von  Frey  (1904)  to  contain 
some  200,000,000,000  molecules  of  mercaptan. 

Passy  (1892a,  1892b)  has  made  similar  minimum  de- 
terminations for  a  number  of  substances  and  has  shown 
that  artificial  musk,  probably  the  most  powerful  of  all 
known  odorous  materials,  is  about  a  thousand  times 
stronger  than  natural  musk.  In  his  other  determinations 
he  found  that  olfactory  acuity  ranged  in  thousandths  of 
a  milligram  per  liter  of  air  from  camphor  at  5  to  vanillin 
at  from  0.005  to  0.0005.  The  last  determination  may  be 
expressed  as  equivalent  to  1/2,000,000,000  of  a  milligram 
of  vanillin  in  a  cubic  centimeter  of  air,  a  high  dilution 
but  still  not  so  extreme  as  that  already  recorded  by 
Fischer  and  Penzoldt  for  mercaptan. 

The  details  of  the  more  important  of  Passy 's  deter- 
minations are  given  in  the  following  table  in  which  ol- 
factory acuity,  as  measured  by  the  minimum  amount  of 
substance  that  was  stimulating  to  the  several  persons 
tested,  is  expressed  in  thousandths  of  a  milligram  per 
liter  of  air. 

TaUe  I. 

Minimum  concentrations   for  olfaction   in  thousandths  of  a  milligram   of 
suhstance  per  liter  of  air  (Passy,  1892b). 

Substances  Thousandths  of  a  milligram 

Camphor 5. 

Ether    1. 

Citral 0.5       to  0.1 

Heliotropin    0.1       to  O.u."> 

Cumarin   0.05     to  0.01 

Vanillin    0.005  to  0.0005 

Passy  (1892c)  has  also  determined  the  minimum  con- 


centration  necessary  for  the  olfaction  of  a  number  of 
alcohols.  These  determinations  have  been  recorded  in 
millionths  of  a  gram  per  liter  of  air  and  are  given 
in  Table  II. 

Passy 's  determinations  indicate  that  the  lower  alco- 
hols have  relatively  faint  odors,  but  that  the  higher 

Table  II. 

Minimum  concentrations  for  olfaction,  in  millionths  of  a  gram  of  alcohol 

per  liter  of  air    (Passy,   1892e). 

Alcohol  Primary                       Secondary           Tertiary 

Methyl 1000 

Ethyl 250 

Propyl 10  to  5                       40 

Normal  Butyl 1                                                20  to  10 

Isobutyl 1 

Normal    Amyl 40  to  20 

Active  sinistral  Amyl 0.6 

Inactive  Isoamyl 0.1 

Caprylic 0.005 

members  of  the  series  are  fairly  comparable  with,  for 
instance,  the  essential  oils.  A  determination  for  ethyl 
alcohol  by  Parker  and  Stabler  in  1913  showed  that  this 
alcohol  could  be  detected  only  to  a  concentration  of  about 
5.75  milligrams  of  alcohol  per  liter  of  air.  The  smaller 
amount  found  by  Passy,  namely  0.25  milligrams  per  liter 
of  air,  is  believed  by  these  authors  to  be  due  to  odorous 
impurities  that  were  found  by  them  in  certain  ethyl 
alcohols  and  that  may  have  been  present  in  those  tested 
by  Passy. 

Some  of  the  more  striking  determinations  by  Allison 
and  Katz  (1919)  are  reproduced  in  Table  III. 

Here  it  will  be  noted  that  the  most  active  mercaptan 
tested,  propyl  mercaptan,  is  detectable  at  a  concentra- 



tion  of  0.006  milligrams  per  liter  of  air  which  is  equal 
to  6/1,000,000  of  a  milligram  per  cubic  centimeter. 
This  determination  is  by  no  means  so  extreme  as  that 
of  Fischer  and  Penzoldt,  1/23,000,000,000  of  a  milligram 
per  cubic  centimeter.  Whether  this  difference  is  due  to  a 
difference  in  the  compounds  used,  for  Fischer  and 

Table  III. 
Concentrations  in  Milligrams  of  Chemical  per  liter  of  air. 


Intensity  of  Odor 





Very  Strong 

Ethyl  ether  












Ethyl  acetate  

Ethyl  mercaptan   .  . 


Oil  of  peppermint  


Methyl  isothiocyanate  .  .  . 
Butyric  acid  

Allyl  isothiocyanate  

Propyl  mercaptan  

Amyl  thioether  

Artificial  musk  

Penzoldt  do  not  state  what  mercaptan  they  tested,  or 
whether  it  represents  a  difference  in  the  methods  em- 
ployed cannot  be  stated.  In  the  table  from  the  work  of 
Allison  and  Katz,  as  in  all  previous  sets  of  determination, 
artificial  musk  is  shown  to  be  without  question  the  most 
stimulating  substance  tested  and  thus  stands  at  the  head 
of  olfactory  stimuli. 

Notwithstanding  the  numerous  discrepancies  between 
the  various  sets  of  determinations  for  olfactory  acuity 
made  by  various  workers,  it  must  be  admitted  that  olfac- 


tion  is  accomplished  through  very  small,  often  infinitesi- 
mally  small,  amounts  of  material,  and  yet  these  amounts 
involve  immensely  large  numbers  of  molecules  of  the 
odorous  substance. 

4.  Physical  Condition  of  Stimulus,  Gas  or  Solution! 
In  olfaction  in  the  air-inhabiting  vertebrates  the  stimu- 
lus has  been  generally  assumed  to  be  material  particles 
in  a  vaporous  or  gaseous  condition  and  not,  for  instance, 
in  the  form  of  a  solution. 

This  opinion  was  long  ago  supported  by  the  experi- 
ments of  Tourtual  (1827)  and  especially  of  Weber  (1847) 
both  of  whom  believed  that  it  could  be  shown  that  sub- 
stances that  could  be  smelled  as  vapors  could  not  be 
smelled  as  solutions  when  introduced  as  such  into  the  nose. 
Thus  Weber  was  unable  to  recognize  cologne  water  when 
this  liquid,  much  diluted  with  ordinary  water,  was  poured 
into  his  nasal  cavities.  He,  therefore,  concluded  that 
though  the  vapor  from  cologne  water  was  easily  smelled, 
a  solution  of  it  was  not  so  sensed  and  that  hence  the 
vaporous  state  of  the  substance  was  necessary  as  a  stim- 
ulus for  the  olfactory  organ.  This  conclusion  was  ac- 
cepted by  a  number  of  investigators  including  Nagel 
(1894,  1904),  Zwaardemaker  (1895),  Haycraft  (1900) 
and  others. 

Aronsohn,  in  1884,  pointed  out  the  great  influence 
that  water  and  temperature  had  on  the  olfactory  organ. 
Ordinary  cold  water  when  introduced  into  the  nose  will 
so  affect  the  organ  of  smell  that  olfaction  is  impossible 
for  some  time  to  come.  Cold  water  is  known  to  excite 
an  increased  production  of  mucous  whose  volume  would 
materially  interfere  with  stimulation  by  covering  up  the 
olfactory  surfaces.  Moreover  if  the  action  of  water  on 


the  organ  of  smell  in  an  air-inhabiting  vertebrate  is 
continued  for  some  time,  it  is  said  to  result  ultimately 
in  the  destruction  of  the  olfactory  hairs.  Thus  Schultze 
(1862)  noted  that  when  the  olfactory  membrane  of  an  air- 
inhabiting  amphibian  is  flooded  with  water,  the  cilia  with 
which  it  is  provided  may  continue  to  beat  for  hours, 
but  the  much  longer  and  heavier  olfactory  hairs  vanish 
almost  at  once.  To  minimize  this  deleterious  effect  Aron- 
sohn,  therefore,  introduced  into  the  nose  material  dis- 
solved, not  in  ordinary  water,  but  in  physiological  salt 
solution  and  at  an  appropriate  temperature.  With  these 
precautions  he  claimed  that  it  was  very  easy  to  recognize 
weak  solutions  of  clove  oil.  Vaschide  in  1901  confirmed 
Aronsohn's  results  and  pointed  out  that  temperature 
was  a  more  important  factor  in  carrying  out  conclusive 
tests  than  the  composition  of  the  solvent. 

These  results,  which  were  in  direct  opposition  to  those 
of  Weber,  were  criticized  by  Zwaardemaker  (1895)  and 
especially  by  Veress  (1903)  who  showed  that  the  pro- 
cedure employed  by  Aronsohn  probably  resulted  in  a 
failure  to  fill  the  olfactory  cleft.  Veress  maintained  that 
unless  great  care  was  taken  at  this  step,  air  was  very 
likely  to  remain  in  this  cleft  and  thus  the  solution  that 
was  being  tested  would  never  really  reach  the  olfactory 
terminals.  Under  such  circumstances  odorous  particles 
would  escape  from  the  solution  into  the  air  filling  the  cleft 
and  thus  reach  the  olfactory  organ  as  in  ordinary  olfac- 
tion.  Thus  it  became  necessary  in  making  a  conclusive 
test  to  take  steps  to  insure  the  complete  filling  of  the 
olfactory  cleft  with  the  solution  to  be  tested.  After 
some  experimentation  on  the  human  cadaver,  Veress  per- 
fected a  technique  whereby  this  could  be  accomplished. 


On  thus  introducing  odorous  solutions  into  the  nasal 
chambers  of  a  living  subject,  he  found  that  these  solutions 
were  stimuli  for  the  olfactory  organs,  but  that  they  did 
not  produce  the  sensation  ordinarily  associated  with 
them.  A  person,  however,  could  soon  learn  to  associate 
a  given  sensation  with  a  particular  substance  and  could 
thus  acquire  an  ability  to  recognize  this  substance,  but  not 
by  what  would  be  called  its  proper  odor.  Veress,  there- 
fore, concluded  that  though  solutions  of  odorous  materials 
are  stimuli  for  the  olfactory  organs,  they  are  inadequate 
rather  than  adequate  stimuli.  It  thus  appears,  contrary 
to  the  results  obtained  by  Weber,  that  the  olfactory 
organs  of  an  air-inhabiting  vertebrate  can  be  stimulated 
by  ordinary  solutions,  though  this  form  of  stimulation 
cannot  be  looked  upon  as  normal. 

To  deny  that  the  olfactory  organs  of  man  and  other 
like  vertebrates  are  stimulated  by  solutions,  as  has  been 
done  by  a  number  of  workers,  implies  a  certain  lack  of 
appreciation  of  the  actual  environment  of  the  olfactory 
terminals.  These  are  the  olfactory  hairs  that  project  in- 
to the  coating  of  mucous  that  covers  the  olfactory  mem- 
brane. These  hairs  appear  to  be  completely  covered  by 
the  mucus  and  should  any  of  their  lash-like  ends  reach  to 
the  outer  surface  of  this  layer,  they  are  certainly  far  too 
delicate  to  project  into  the  adjacent  air;  they  would 
unquestionably  remain  within  the  limits  of  the  mucous 
layer.  Thus  the  olfactory  hairs  are  at  all  times  sur- 
rounded by  watery  mucous,  which  is  in  contact  on  its  outer 
face  with  the  air  carrying  the  odorous  particles.  These 
particles,  as  already  indicated,  must  be  caught  in  great 
numbers  on  the  moist  mucous  surface,  absorbed  according 
to  Zwaardeniaker  (1918b),  and,  since  they  are  in  the  form 


of  gaseous  or  vaporous  particles,  they  probably  enter 
quickly  into  solution  in  the  watery  mucous  and  in  this  state 
come  in  contact  with  the  olfactory  hairs.  From  the  nature 
of  the  surroundings,  then,  it  would  seem  extremely  im- 
probable that  the  stimulating  material  for  the  olfactory 
terminals  should  be  in  any  other  state  than  that  of  a  solu- 
tion. This  opinion  seems  to  be  gaining  ground  rapidly 
among  the  more  recent  workers,  for  it  has  found  clear 
expression  within  the  last  few  years  in  papers  by  Back- 
man  (1917a),  by  Durand  (1918b),  and  in  a  qualified  way 
by  Henning  (1916). 

As  already  indicated,  the  difficulty  met  with  in  at- 
tempting to  stimulate  adequately  the  human  olfactory 
epithelium  with  solutions  of  odorous  material  is  due  in 
all  probability  to  the  effects  of  the  solvent  on  the  olfac- 
tory hairs  and  not  to  the  incapacity  of  these  terminals  to 
be  stimulated  by  solutions.  These  hairs  are  apparently 
very  delicately  attuned  to  a  mucous  environment  that 
would  be  very  difficult  to  duplicate  experimentally  and  yet 
this  environment  seems  to  be  essential  to  a  wholly  suc- 
cessful test.  Care  as  to  temperature  and  salt  contents  of 
the  solvent,  as  emphasized  by  Aronsohn,  Vaschide,  and 
Veress,  are  probably  only  the  first  steps  in  this  direction. 

The  relation  of  the  solubility  of  a  substance  to  its 
efficiency  as  an  olfactory  stimulus  has  been  discussed 
recently  by  Backman  (1917a).  This  investigator  has 
expressed  the  opinion  that  not  only  the  aqueous  environ- 
ment of  the  olfactory  hairs  must  be  considered  but  also 
the  substance  of  the  hairs  themselves.  This  he  believes 
to  be  lipoid  in  character,  an  opinion  that  is  supported  by 
the  well  known  fact  that  these  hairs  are  best  demon- 
strated by  osmic  acid.  If  the  embedding  mucous  layer  is 


watery  and  the  olfactory  hairs  oily,  it  follows  that  any 
substance  that  gains  entrance  into  the  body  of  the  hair 
must  first  have  been  dissolved  in  water  and  then  in  oil. 
From  this  standpoint  Backman  attempted  to  determine 
whether  there  was  any  relation  between  the  effectiveness 
of  certain  olfactory  stimuli  and  their  solubility  in  water 
and  in  oil.  Water  and  olive  oil,  each  at  30  degrees  centi- 
grade, were  used  as  the  test  solvents.  Thus  methyl  alco- 
hol and  ethyl  alcohol,  which  are  without  strong  odor, 
were  found  to  be  freely  soluble  in  water,  but  only  very 
slightly  soluble  in  oil.  Hence  while  they  would  dissolve 
abundantly  in  the  olfactory  mucous,  they  would  fail  to 
enter  the  hairs  to  any  great  extent  in  consequence  of  which 
their  effectiveness  as  stimuli  must  be,  according  to  Back- 
man, very  slight.  On  the  other  hand  normal  butyl  alco- 
hol has  a  strong  odor  and  its  efficiency  as  a  stimulus  was 
believed  to  depend  upon  the  fact  that  it  is  soluble  in  water 
to  the  extent  of  8.3  per  cent  and  in  oil  to  an  almost  indefi- 
nite amount.  Other  substances  showed  somewhat  different 
relations.  Thus  chlorbenzol  could  be  detected  at  a  dilu- 
tion of  6.7  x  10."*  gram-molecules  per  liter  of  air,  and  is 
soluble  in  water  to  the  extent  of  0.25  per  cent  and  in  oil 
indefinitely.  Brombenzol  could  be  smelled  at  the  some- 
what greater  dilution  of  1.1  x  10.~8  gram-molecules  per 
liter  of  air;  yet  it  is  less  soluble  in  water  (0.045  per  cent) 
than  chlorbenzol  though  indefinitely  soluble  in  oil.  In 
these  instances  the  degrees  of  solubility  in  water  are  the 
reverse  of  the  effectiveness  of  these  two  substances  as 
olfactory  stimuli.  Possibly  solubility  in  oil,  as  intimated 
by  Larguier  des  Bancels  (1912),  is  of  much  more  signifi- 
cance for  olfactory  stimulation  than  solubility  in  water. 
If  the  olfactory  hairs  in  man  are  provided  with  flagella, 


such  as  have  been  described  by  Jagodowski  (1901)  in  the 
pike,  and  the  distal  ends  of  these  flagella  reach  through 
the  olfactory  mucous  to  the  nasal  atmosphere,  the  odor- 
ous particles  may  come  directly  in  contact  with  them 
and  dissolve  in  their  lipoid  substance  without  pass- 
ing through  an  intermediate  watery  layer.  In  that  case 
solubility  in  lipoid  would  be  the  only  form  of  solubility 
necessary  for  the  introduction  of  an  effective  stimulus. 
That  a  number  of  odorous  substances  are  more  soluble 
in  lipoid  than  in  water  has  recently  been  shown  by  Kremer 
(1917)  who  found  that  larger  quantities  of  citral. 
guaiacol,  pyridine,  and  even  chloroform  and  ether  would 
dissolve  in  a  saturated  aqueous  solution  of  lecithin  than 
in  pure  water.  Of  course  the  varying  capacity  for  re- 
action of  such  materials  as  may  thus  become  dissolved 
in  the  substance  of  the  hairs  must  profoundly  influence 
stimulation  and  possibly  it  is  in  this  direction  that  the 
difference  between  such  substances  as  chlorbenzol  and 
brombenzol  is  to  be  explained.  But  however  these  de- 
tails may  be  worked  out  eventually,  the  general  opinion 
that  olfactory  stimulation  is  dependent  upon  some  form 
of  solution  seems  to  be  beyond  question. 

That  the  material  thus  dissolved  must  act  chemically 
on  the  olfactory  receptors  and  not  by  means  of  any  radia- 
tion that  it  may  give  out  seems  probable  from  the  fact 
that  olfactory  stimuli  are  substances  that  are  not  known 
to  be  radio-active.  That  there  is  a  kind  of  physiological 
radio-activity,  such  as  has  been  claimed  recently  for  po- 
tassium by  Zwaardemaker  (1918a,  1920)  and  as  might  be 
urged  for  the  materials  of  olfactory  stimulation,  seems 
extremely  improbable  from  the  recent  work  of  R.  F.  Loeb 
(1920)  and  of  J.  Loeb  (1920).  Moreover  it  would  be 


very  difficult  to  explain  the  variety  of  olfactory  sensa- 
tions on  the  basis  of  stimulation  by  radio-activity,  but 
the  assumption  that  the  stimuating  materials  act  chemi- 
cally on  the  substance  of  the  receptor  is  in  easy  accord 
with  the  diversity  of  olfactory  experience. 

5.  Olfaction  in  Fishes.  It  has  already  been  pointed 
out  that  most  fishes  possess  paired  olfactory  sacs  whose 
structure  and  innervation  are  essentially  indentical  with 
the  corresponding  parts  in  the  air-inhabiting  vertebrates. 
Nevertheless  currents  of  water  flow  through  these  sacs 
and  such  stimulation  as  they  receive  must  come  from 
these  currents.  Nagel  (1894),  who  was  one  of  the  most 
vigorous  opponents  to  the  idea  that  the  olfactory  organs 
were  stimulated  by  solutions  and  believed  that  gases  or 
vapors  were  the  only  real  stimuli  for  these  receptors, 
was  led  to  conclude  that  the  so-called  olfactory  organs  of 
fishes  were  fundamentally  different  from  those  of  the 
air-inhabiting  forms  and  that  they  probably  more  nearly 
resembled  organs  of  taste  than  any  other  receptor  pos- 
sessed by  the  higher  animals.  This  opinion  was  based 
upon  theoretic  considerations  rather  than  upon  any  par- 
ticular observation  or  test. 

But  before  these  views  had  been  expressed  by  Nagel, 
a  certain  amount  of  experimental  evidence  concerning 
olfaction  in  fishes  had  been  gathered.  This  was  prelimi- 
nary in  character  and  inconclusive,  but  it  nevertheless 
paved  the  way  for  further  advance.  Thus  the  observa- 
tion of  Aronsohn  (1884a),  that  goldfish,  which  ordinarily 
will  eat  ant  pupae  with  avidity,  would  not  take  these  pupae 
after  they  had  been  smeared  with  a  little  oil  of  cloves, 
is  not  final  evidence  that  the  fish  scented  the  oil,  for 
it  is  entirely  possible  that  this  oil  irritated  the  skin  of  the 


fish's  snout  and  did  not  stimulate  the  olfactory  apparatus 
at  all.  Nor  was  the  discoveiy  made  by  Steiner  (1888), 
that  the  spontaneous  appropriation  of  food  by  the  shark, 
Scyllium,  ceases  on  the  removal  of  the  cerebral  lobes  or 
simply  on  cutting  the  connection  between  these  lobes  and 
the  olfactory  bulbs,  satisfactory  evidence  that  the  olfac- 
tory apparatus  in  these  fishes  is  an  organ  of  smell  rather 
than  a  receptor  for  taste  or  some  closely  allied  sense. 
Nagel  (1894)  noted  that  the  front  of  the  head  of  the  fish, 
Barbus,  was  as  sensitive  to  sapid  substances  after  the 
olfactory  tracts  had  been  cut  as  before  that  operation,  and 
Sheldon  (1909),  who  studied  the  dogfish  with  great  ful- 
ness, demonstrated  that  the  decided  sensitiveness  of  the 
nostrils  of  this  fish  to  weak  solutions  of  oil  of  cloves, 
pennyroyal,  thyme,  and  the  like,  was  not  influenced  by 
severing  the  olfactory  crura,  but  disappeared  on  cutting 
the  combined  maxillary  and  mandibular  branches  of  the 
trigeminal  nerve.  Evidently  the  nasal  surfaces  of  fishes 
like  those  of  the  higher  vertebrates,  are  innervated  by 
fibers  from  the  trigeminal  nerve,  and  it  is  this  nervous 
mechanism  rather  than  the  true  olfactory  apparatus,  that 
is  stimulated  by  the  substances  that  have  ordinarily  been 
applied  by  experimenters.  In  1909,  Baglioni  showed  that 
blinded  fishes  were  excited  by  the  presence  of  food.  None 
of  these  experiments,  however,  demonstrated  conclu- 
sively that  smell  rather  than  taste  or  some  other  allied 
sense,  was  concerned  as  the  receptor. 

As  early  as  1895  von  Uexkull  observed  that  dogfishes 
from  which  the  olfactory  membranes  had  been  removed 
did  not  respond  to  the  presence  of  food  whereas  normal 
dogfishes  three  to  five  minutes  after  food  had  been  in- 
troduced into  their  tank,  sought  it  with  great  eagerness. 


In  these  experiments  no  attempts  were  made  to  exclude 
sight  or  to  ascertain  the  effects  of  the  operation.  In  ex- 
periments carried  out  by  me  in  1910  an  attempt  was 
made  to  gain  more  conclusive  evidence.  Five  normal 
catfishes  (Amiurus)  were  allowed  to  swim  in  an  aqua- 
rium in  which  were  hung  two  wads  of  cheese  cloth  one 
containing  concealed  earthworms,  and  the  other  made 
of  cloth  only.  In  the  course  of  an  hour  the  wad  con- 
taining the  worms  was  seized  eleven  times  by  the  fishes 
notwithstanding  the  fact  that  from  time  to  time  this  wad 
was  interchanged  in  position  with  the  other.  During  the 
same  period  the  wad  without  worms  was  passed  over  by 
the  fishes  many  times  and  never  excited  any  noticeable 

Ten  catfishes  were  next  prepared  for  further  experi- 
mentation; in  five  of  these  the  olfactory  tracts  were  cut 
and  from  the  remaining  five  the  barbels,  the  seat  of  the 
chief  external  gustatory  organs,  were  removed.  After 
the  fishes  had  recovered  from  these  operations,  they  were 
put  in  an  aquarium  into  which  was  introduced  a  wad  of 
cheescloth  containing  minced  earthworms.  During  the 
first  hour  the  wad  was  seized  34  times  by  fishes  without 
barbels  but  with  normal  olfactory  organs  and,  though 
often  passed  over  by  fishes  with  cut  olfactory  tracts,  it 
was  never  seized  by  any  of  these  and  " nosed"  only  once 
by  one  of  them.  None  of  these  fishes  paid  any  attention 
to  a  wad  of  cloth  containing  no  worms.  Repetitions  of 
these  tests  gave  uniformly  similar  results  and  led  to  the 
conclusion  that  the  olfactory  organs  of  the  catfish  are 
serviceable  in  sensing  food  at  a  distance  much  beyond 
that  at  which  the  organs  of  taste  are  capable  of  acting; 
in  other  words,  catfishes  truly  scent  their  food. 


Similar  experiments  on  the  killifish  (Fundulus)  gave 
like  results  Parker  (1911).  Here,  however,  the  olfac- 
tory organs  were  excluded,  not  by  cutting  the  olfactory 
tracts,  but  by  stitching  up  the  anterior  nares.  As  a  re- 
sult of  this  operation  the  fish  no  longer  responded  to  hid- 
den food,  but  quickly  reacquired  this  power  after  the 

Fia.  15. — Ventral  view  of  the  head  of  a  Hammer-head  Shark  (Cestracion)  showing  the 
olfactory  pita  (o)  widely  separated.    After  Carman,  1913,  Plate  1,  Fig.  2. 

anterior  nares  had  been  reopened.  These  results  were 
confirmed  in  work  on  the  dogfish,  (Mustelus),  by  Sheldon 
(1911)  and  on  the  swellfish,  (Spheroides),  by  Copeland 
(1912).  Sheldon  closed  the  nares  of  the  dogfish  with 
cotton  plugs  and,  in  1914,  I  showed  that  when  only  one 
nostril  is  thus  plugged,  the  fishes  turn  persistently  to- 
ward the  side  of  the  open  nostril.  Such  responses 
indicate  that  in  the  seeking  of  food  under  normal  con- 
ditions, dogfishes,  and  probably  other  fishes  as  well,  turn 


toward  the  side  on  which  the  concentration  of  odorous 
particles  is  greater.  The  certainty  of  this  operation 
would  increase  in  proportion  as  the  nostrils  of  a  given 
fish  are  separated  one  from  the  other  laterally.  A  good 
example  of  an  animal  in  which  this  condition  reaches 
its  maximum  is  seen  in  the  hammerhead  shark  in  which 
the  nostrils,  as  well  as  the  eyes,  are  carried  on  the  re- 

Table  IV. 

Records  in  per  cent,  of  Turning  Movements  of  three  Dogfish  under  the  follow- 
ing successive  Conditions:  Normal,  Left  Nostril  Occluded,  Right  Nostril 
Occluded,  Both  Nostrils  Open  Parker,  (1914). 

States  of  Fishes 

Turning  Movements  in  Per  Cents. 

To  left 

To  right 





Left  nostril  occluded  

Right  nostril  occluded  

Both  nostrils  open  

markable  lateral  projections  that  extend  sidewise  from 
its  head  into  the  sea  (Fig.  15). 

The  turning  response  of  dogfishes  under  the  condi- 
tions mentioned  in  the  preceding  paragraph  has  a 
striking  resemblance  toi  the  circus  movements  in  the 
tropic  reactions  of  many  of  the  lower  animals  and,  were 
it  not  that  fishes  are  so  highly  organized,  it  might  be 
accepted  at  once  as  a  response  of  that  kind.  The  detailed 
condition  of  such  reactions  is  well  illustrated  by  the 
records  in  Table  IV. 

As  a  result  of  the  evidence  thus  far  accumulated,  it 
seems  quite  clear,  contrary  to  the  opinions  expressed  by 
Nagel  and  others,  that  many  fishes  scent  their  food  much 
as  air-inhabiting  animals  do  and  that  they  must  be  re- 


garded  as  possessing  powers  of  olfaction  fairly  compar- 
able with  those  of  the  higher  forms.  This  opinion  is  in 
entire  harmony  with  the  well  known  fact  that  fishes, 
especially  sharks,  can  be  drawn  from  a  long  distance  by 
ill  smelling  bait  or  by  oily  fish  carcasses  ground  up  and 
thrown  into  the  water  as  in  the  practice  of  chumming. 
The  extremely  small  amount  of  substance  needed  in  these 
operations  agrees  well  with  what  is  known  of  olfaction 
among  air-inhabiting  vertebrates  and  reaches  almost  in- 
finitesimal proportions  as  is  indicated  by  the  work  of 
Olmstcd  (1918)  on  Amiurus. 

The  water-inhabiting  stages  of  amphibians  will  doubt- 
less be  found  to  exhibit  the  same  type  of  olfaction  as 
that  seen  in  fishes.  This  is  already  clearly  indicated  by 
the  work  of  Copeland  (1913)  on  the  newt  Diemyctylus 
and  of  Kisser  (1914)  on  tadpoles. 

The  opinion  that  fishes  possess  powers  of  olfaction 
comparable  with  those  of  the  air-inhabiting  vertebrates, 
though  rejected  by  many  of  the  older  writers,  has  been 
accepted  in  recent  years  by  Baglioni  (1913)  and  by 
Luciani  (1917).  In  fishes  there  can  be  no  doubt  that  tho 
stimulating  material  for  the  olfactory  organs  is  carried 
in  the  current  of  water  that  is  passing  more  or  less  con- 
tinuously through  these  parts.  Since  in  air-inhabiting 
vertebrates  the  stimulating  materials  are  caught  on  the 
watery  mucous  of  the  olfactory  surfaces,  it  follows  that, 
as  Durand  (1918b)  has  recently  declared,  the  olfactory 
stimulus  throughout  the  whole  range  of  vertebrates  is 
material  in  a  state  of  solution  and  not  simply  a  gas  or 
a  vapor.  This  conclusion  is  in  agreement  with  the 
opinion  expressed  many  years  ago  by  Johannes  Mu'ller. 
Henning  (1916),  some  time  since  called  attention  to  the 


possibility  that  odorous  material  may  form  with  the  ol- 
factory mucous  an  emulsion  rather  than  a  true  solution, 
but  this  suggestion  did  not  seem  even  to  Henning  to  be 
of  much  significance,  for  in  other  parts  of  his  work  he 
refers  repeatedly  to  the  state  of  the  stimulating  material 
as  that  of  a  solution  and  there  appears  to  be  no  good 
ground  for  assuming  that  such  is  not  the  case. 

6.  Fatigue  and  Exhaustion.  It  is  well  known  that 
the  olfactory  organs  in  man  are  quickly  and  easily  fa- 
tigued by  continuous  exposure  to  odorous  materials. 
Persons  whose  occupations  lead  them  to  work  among 
disagreeable  odors  soon  become  insensitive  to  these  and 
it  has  long  been  recognized  that  invalids  are  not  affected 
by  the  malodors  that  may  come  from  their  own  bodies. 
Although  these  conditions  of  irresponsiveness  may  be 
due  in  part  to  central  nervous  states  such  as  lack  of  at- 
tention and  the  like,  they  are  also  dependent  in  part 
on  peripheral  exhaustion.  The  effects  of  unpleasant 
smells  on  the  growth  of  guinea  pigs  has  been  tested  very 
recently  by  Winslow  and  Greenberg  (1918).  These  in- 
vestigators employed  a  pair  of  air-proof  cages  through 
which  were  passed  1.5  cubic  feet  of  air  per  minute 
amounting  to  4  liters  of  air  per  minute  for  each  animal 
in  the  test.  Through  one  of  these  cages  pure  air  was 
circulated;  through  the  other,  air  that  had  passed  over 
fresh  moist  faeces  and  that  in  consequence  was  impreg- 
nated with  a  strong  faecal  odor.  A  total  of  15  sets  of 
growing  guinea  pigs,  including  261  animals,  were  sub- 
jected to  these  conditions.  In  the  first  week  of  the  tests 
the  animals  supplied  with  faecal  air  did  not  grow  as  much 
as  the  controls  did,  but  in  the  second  week  they  caught 
up  in  weight  with  the  controls  and  were  thereafter  in- 


distinguishable  from  them.  Thus  the  guinea  pig,  like 
man,  though  sensitive  to  disagreeable  odors  in  the  be- 
ginning, appears  to  become  in  the  course  of  time  entirely 
inert  to  this  form  of  stimulation. 

To  test  the  immediate  effects  of  the  continuous  action 
of  odorous  substance  on  the  olfactory  organ  of  man, 
Aronsohn  (1884a)  determined  the  length  of  time  certain 
olfactory  stimuli  at  full  strength  continued  to  call  forth 
sensation.  Thus  oil  of  lemon  and  oil  of  orange  were 
smelled  by  nine  persons  till  the  odors  of  these  substances 
could  no  longer  be  perceived  The  period  necessary  to 
bring  about  this  obliteration  of  sensation  varied  from 
2.5  minutes  to  11  minutes  with  an  average  of  3  minutes. 
A  0.2  per  cent  solution  of  cumarin  in  water  was  smelled 
for  from  1.75  to  2.3  minutes  after  which  it  was  no  longer 
sensed.  Thus  olfactory  exhaustion  under  strong  stimu- 
lation is  accomplished  in  a  very  few  minutes.  The  re- 
covery of  excitability  is  apparently  equally  rapid  and 
may  be  accomplished  in  as  short  a  time  as  from  1  to  3 
minutes  though  complete  recovery  probably  requires  a 
longer  time. 

Zwaardemaker  (1895)  tested  fatigue  in  another  way 
and  determined  by  means  of  his  olfactometer  the  in- 
crease in  minimum  stimulation  as  the  olfactory  organ 
gradually  approximated  exhaustion.  During  a  continu- 
ous stimulation  of  known  intensity  the  minimum  stimu- 
lus was  from  time  to  time  determined  and  was  found  to 
increase  steadily.  Two  substances,  benzoin  and  rubber, 
at  two  different  strengths  were  tested  (Fig.  16).  Ben- 
zoin induced  fatigue  more  rapidly  than  rubber  and  of 
the  two  concentrations  employed  for  each  substance  the 



stronger   in   each  instance   called   forth   fatigue   more 
quickly  than  the  weaker. 

Some  persons  are  absolutely  devoid  of  true  olfaction, 
a  condition  which,  as  already  pointed  out,  is  attendant 
upon  certain  deficiencies  in  the  essentials  of  the  olfac- 
tory apparatus  and  which  is  designated  as  anosmia. 



0      10     20     30      40     50     60      70     80      90  Sec* 

FIG.  16. — Curves  of  olfactory  exhaustion  produced  by  the  action  of  benzoin  of  9  and 
of  3.5  olfacties  and  &by  rubber  of  14  and  of  10  olfacties,  acting  for  different  periods. 
The  threshold  values  in  olfacties  are  marked  on  the  ordinates  and  the  duration  of 
stimulation  in  seconds  on  the  abscisste.  After  Zwaardemaker,  1895,  Fig.  22. 

This  state  may  be  congenital  or  acquired  and  acquired 
anosmia  may  be  either  permanent  or  temporary.  Some 
forms  of  anO'Smia,  like  color-blindness,  are  probably  in- 
heritable Glaser  (1918).  Of  considerable  interest  from 
a  theoretical  standpoint  are  the  cases  of  partial  olfac- 
tory defects.  Winkler  noted  a  patient  who  was  quite 
incapable  of  smelling  benzoin  though  he  easily  recognized 
musk  and  another  who  was  just  the  reverse  of  the  first 


one.  Blakeslee  (1918)  has  recorded  similar  cases  in 
relation  to  the  odor  of  verbena  flowers.  Probably  many 
persons  are  defective  in  this  respect  though  their  defects 
may  not  have  been  serious  enough  to  have  attracted 
attention  and  record. 

Temporary  partial  anosmia  may  accompany  certain 
diseases  or  may  be  induced  by  the  application  to  the 
olfactory  surfaces  of  anesthetizing  drugs.  Cocaine  has 
been  used  in  this  way  by  a  number  of  investigators,  in- 
cluding Zwaardemaker,  but  without  very  clearly  defined 
results.  Zwaardemaker  observed  that  temporary  anos- 
mia induced  in  this  way  was  preceded  by  a  brief  period 
of  increased  sensibility  or  hyperosmia.  Subsequently 
Keuter  (1900)  found  that  cocaine  was  also  followed  by 
hyperosmia.  Roljett  (1899)  produced  a  complete  an- 
osmia by  the  use  of  gymnemic  acid  after  which  different 
olfactory  sensa.tions  returned  at  different  intervals. 

7.  Qualities  of  Odors.  The  qualities  of  odors  ap- 
pear to  be  almost  innumerable.  When  we  attempt  to 
name  on  odor,  we  almost  always  designate  it  by  the  body 
from  which  the  odorous  material  emanates  like  the  smell 
of  heliotrope,  of  onion,  of  rubber,  and  so  forth.  With 
tastes,  as  we  shall  see  later,  there  are  at  least  four 
clearly  marked  qualities,  sweet,  sour,  bitter,  and  salty. 
The  first  three  of  these  are  general  terms  connected  in 
no  necessary  way  with  the  substances  associated  with 
them  as  stimuli,  and  we  are  continually  finding  new  sub- 
stances whose  tastes  are  some  one  of  these  three.  The 
odors  of  new  substances,  on  the  other  hand,  are  almost 
certain  to  be  individual  and  novel  and  to  agree  with  odors 
already  known  only  in  a  most  general  way.  Thus  odors 
have  a  certain  historical  value  and  get  their  names  after 
the  introduction  of  the  substances  with  which  they  are 


associated;  the  smell  of  illuminating  gas  was  not  a  gen- 
erally known  odor  till  this  material  was  brought  into 
common  use.  Should  it  be  abandoned  commercially,  its 
odor  would  cease  to  be  a  part  of  common  human  sensa- 
tion. In  consequence  of  economic  changes  many  odors 
of  trade  articles,  of  kitchen  products,  and  the  like  have 
disappeared  from  the  list  of  human  sensations  and  many 
new  ones  have  come  in.  Yet  notwithstanding  this  rela- 
tively rapid  evolution  in  the  field  of  olfaction,  the  organ 
of  smell  seems  to  remain  the  same ;  it  gives  up  old  forms 
of  stimulation  and  takes  on  new  ones  in  a  way  that  is 
almost  incredible.  As  a  result  of  these  peculiarities  of 
the  olfactory  organ  the  classification  of  odors  has 
proved  to  be  a  most  perplexing  problem  and  has 
resulted  in  most  instances  in  what  seem  to  be  extremely 
artificial  schemes. 

Haller  and  particularly  Linnaeus  proposed  systems  of 
odors  that  have  formed  the  bases  for  many  of  the  modern 
classifications  such  as  the  one  given  by  Zwaardemaker 
(1895).  In  this  odors  are  arranged  in  nine  general 
classes  each  of  which  may  contain  two  or  more  sub- 
divisions. These  nine  classes  are  briefly  as  follows : 

1.  Etherial  odors;  three  subdivisions:  odors  of  fruits, 

beeswax,  ethers. 

2.  Aromatic  odors ;  five  subdivisions :  odors  of  camphor, 

cloves,  lavender,  lemon,  bitter  almond. 

3.  Balsamic  odors ;  three  subdivisions :  odors  of  flowers, 

violet,  vanilla,  cumarin. 

4.  Ambrosial  odors ;   two   subdivisions :   odors   of  am- 

ber, musk. 

5.  Alliaceous  odors ;  three  subdivisions :  odors  of  hydro- 

gen sulphide,  hydrogen  arsenide,  chlorine. 


6.  Empyreumatic   odors;   two    subdivisions:    odors   of 

roast  coffee,  benzole. 

7.  Caprilic  odors;  two  subdivisions:  odors  of  cheese, 

rancid  fat. 

8.  Repulsive  odors;  two  subdivisions:  odors  of  deadly 

nightshade,  bedbug. 

9.  Nauseating  odors;  two  subdivisions:  odors  of  car- 

rion, faBces. 

A  survey  of  this  classification  shows  at  once  that  more 
or  less  of  it  is  associative  and  subjective  and  hence  ar- 
tificial, for  what  may  be  repulsive  to  one  person  may  be 
just  the  reverse  to  another.  It  is,  therefore,  not  sur- 
prising that  some  of  the  recent  students  of  this  subject, 
as  for  instance  Henning  (1916),  have  advised  the  com- 
plete abandonment  of  such  arrangements  and  have 
sought  to  establish  by  a  thorough  re-testing  of  odors  an 
impersonal  and  reasonable  classification.  As  the  re- 
sult of  an  extended  and  judicious  re-examination  of  odors 
Henning  has  come  to  the  conclusion  that  they  fell  into 
six  fundamental  classes  as  follows : 

1.  Spicy  odors,  such  as  those  of  fennel,  sassafras  oil, 

anise,  and  cloves. 

2.  Flowery  odors,  such  as  those  of  heliotrope,  cumarin, 

and  geranium  oil. 

3.  Fruity  odors,  such  as  those  of  oil  of  orange,  citro- 

nella,  oil  of  bergamot,  and  acetic  ether. 

4.  Resinous  or  balsamic  odors,  such  as  those  of  tur- 

pentine, of  Canada  balsam,  and  of  eucalyptus  oil. 

5.  Burnt  odors,  such  as  those  of  tar  and  pyridine. 

6.  Foul  odors,  such  as  those  of  carbon  bisulphide  and 

of  hydrogen  sulphide. 
Although   each    of   these    six   classes,   according   to 



Henning,  is  represented  by  a  number  of  odors,  it  is  not  ab- 
solutely separated  from  the  others,  but  between  any  pair 
of  them  there  are  numerous  odors  that  assume  interme- 
diate positions.  The  six  classes,  however,  are  the  striking 
predominant  elements  in  this  complex  and  are  in  no  sense 
submerged  in  the  general  array  of  odors. 

Henning  has  tried  to  make  clear  his  idea  of  the  rela- 
tions of  these  six  classes  by  imagining  them  located  one 







FIQ.  17. — Olfactory  prism.    After  Henning,  1916,  Fig.  4. 

at  each  corner  of  a  three-sided  prism  which  he  calls  the 
olfactory  prism  (Fig.  17).  From  each  corner  of  this 
prism  lines  may  be  imagined  to  pass  out  to  the  other 
corners ;  these  lines  traverse  either  the  edges  of  the  prism 
or  pass  over  its  faces  and  mark  the  positions  of  all  in- 
termediate odors.  Thus  all  odors,  be  they  fundamental 
or  intermediate,  find  places  on  the  surface  of  the  prism. 
Relations  indicated  by  lines  within  the  prism  and  con- 
necting any  two  points  on  its  surface  indicate  only 
mixed  odors.  Thus  by  means  of  a  figure  of  three  dimen- 
sions Henning  brings  into  clear  view  the  relations 
he  conceives  to  exist  between  the  six  fundamental  odors, 


their  intermediates  and  mixtures.  So  far  as  an  arrange- 
ment of  odors  is  concerned  the  clarity  of  Henning's 
scheme  is  at  once  its  most  attractive  and  most  suspi- 
cious feature. 

8.  Chemical  Relations  of  Odors.  The  scientific 
value  of  any  classification  of  odors  will  depend  upon  the 
success  with  which  such  a  classification  brings  the  odor- 
ous substances  as  stimuli  into  relation  with  the  receptor. 
A  satisfactory  classification  ought  to  make  evident  the 
number  of  elements  or  components  concerned  in  olfac- 
tion.  That  olfaction  is  made  up  of  a  number  of  compo- 
nents is  far  from  established,  but  what  may  be  called 
the  component  theory  of  olfacton  is  generally  assumed 
by  the  majority  of  writers  on  this  subject  Zwaardemaker 
(1895).  That  the  classification  outlined  by  Zwaarde- 
maker shows  very  little  of  this  feature  is  readily  admit- 
ted even  by  this  author  himself.  Quite  aside  from  the 
fact  that  it  may  include  irritants  as  well  as  true  odorous 
substances,  its  classes  do  not  stand  up  well  under  experi- 
mental test.  Nagel  (1897)  tested  this  question  in  an 
investigation  of  the  odors  of  vanillin  and  cumarin. 
These  two  substances,  according  to  Zwaardemaker 's 
classification,  belong  not  only  to  the  same  class  of  bal- 
samic odors  but  to  the  same  subdivision,  the  vanilla  odor. 
They  ought,  therefore,  to  show  considerable  olfactory 
similarity.  Nagel  attempted  to  test  this  relationship  by 
ascertaining  whether  the  temporary  exhaustion  of  the  ol- 
factory organ  by  one  of  these  substances  would  influence 
its  receptive  capacity  for  the  other.  To  carry  out  this 
he  prepared  an  aqueous  solution  of  the  two  substances 
in  such  proportions  that  the  smell  of  only  vanillin  could 
be  recognized.  He  then  exhausted  the  olfactory  organ 


for  vanillin  by  smelling  for  a  long  time  a  pure  solution 
of  this  material.  On  testing  now  the  solution  containing 
the  mixture  of  substances,  it  was  found  to  smell  only 
of  cumarin.  Thus  the  exhaustion  of  the  olfactory  sur- 
face for  vanillin  did  not  prevent  stimulation  by  cumarin. 
The  placing  of  these  two  substances  in  the  same  subdi- 
vison  is,  therefore,  obviously  artificial. 

Similar  evidence  as  to  the  artificiality  of  Zwaarde- 
maker's  classifications  had  also  been  obtained  from  the 
study  of  persons  suffering  from  partial  anosmia  and  from 
neither  this  line  of  investigation  nor  from  that  dealing 
with  partial  exhaustion  has  there  come  any  special  jus- 
tification of  the  conventional  olfactory  groupings. 

Yet  it  is  admitted  on  all  sides  that  olfaction  is  essen- 
tially a  chemical  process.  And,  as  a  matter  of  fact, 
some  progress  has  been  made  in  discovering  relations 
between  chemical  structure  and  olfactory  sensation. 
This  isi  not  necessarily  of  a  general  nature,  but  seems/ 
usually  to  be  limited  to  narrow  ranges.  Thus  among  the 
alcohols  Passy  (1892c)  has  discovered  that  the  olfactory 
potency  increases  progressively  in  passing  over  this  se- 
ries from  methyl  to  amyl  as  shown  in  Table  V. 

Ba,ckman  (1917c)  has  likewise  determined  that  ini 
the  methylbenzene  series  olfactory  acuity  for  benzene, 
toluene,  xylene,  cumene,  and  durene  increases  as  the  sub- 
stitute methyl  group  increases. 

Changes  in  the  quality  of  odors  also  follow  some  natu- 
ral series  of  organic  compounds  as  has  been  pointed  out 
by  Hay  craft  (1900)  in  the  following  etherial  salts. 
Ethyl  acetate  with  acetic  etherial  odor. 
Propyl  acetate  with  acetic  odor  and  slight  flavor. 
Butyl  acetate  with  slight  acetic  odor  and  pineapple  flavor. 


Amyl  acetate  with  no  acetic  odor  but  well  marked  pine- 
apple flavor. 

Ethyl  acetate  and  amyl  acetate  have  entirely  distinct 
odors,  but  when  propyl  acetate  and  butyl  acetate  are 
taken  into  consideration  the  four  compounds  form  a  se- 
ries in  which  there  is  a  transition  in  odors  corresponding 

Table  V. 


Estimated  potencies  of  alcohols,  Passy    (1892c). 

Alcohol  Estimated  Potency 

Methyl 1 

Ethyl 4 

Propyl 100 

Butyl 1000 

Amyl 10000 

to  the  changes  in  chemical  structure.  Other  series  of 
homologues,  however,  such  as  the  one  tested  by  Huyer 
(1917),  analine,  o-,  m-,  and  p-toluidine,  xylidine,  and  cu- 
mioline,  show  no  such  relations. 

Not  a  few  investigators  have  suggested  that  the  odors 
of  many  substances  depend  upon  the  number  and  ar- 
rangement of  certain  chemical  radicals  contained  within 
the  odorous  molecule.  Such  radicals  are  commonly 
called  osmophoric  groups.  Perhaps  one  of  the  most 
considerable  studies  of  this  kind  was  that  carried  out  by 
Cohn  (1904),  but  without  commensurate  results.  The 
most  recent  and  ambitious  of  these  attempts  is  by  Hen- 
ning  (1916)  whose  classification  of  odors  has  already  been 
referred  to. 

Henning's  studies  on  the  relations  of  odors  to  chemical 
constitution  have  to  do  almost  entirely  with  the  aromatic 
compounds,  though  there  is  no  reason  to  believe  that  his 
generalizations,  if  true,  may  not  be  extended  eventually 


t   .    •  f 

to  the  aliphatic  series.  He  abandons  the  idea  that  spe- 
cial odors  are  to  be  associated  with  particular  osmopho- 
ric  groups.  In  odors  these  groups  are  significant,  not 
because  of  the  structure  they  themselves  possess,  but 
because  of  the  positions  they  may  occupy  on  the  benzene 
ring.  Osmophoric  groups  are  such  as  the  hydroxyl,  al- 
dehyde, keton,  ester,  nitro,  and  nitril  groups.  None  of 
these,  however,  is  associated  with  a  particular  odor,  but 
any  one  may  be  the  occasion  of  odor,  if  it  occupies  an 
appropriate  place  on  a  benzene  ring.  The  position  on  the 
ring  not  the  particular  radical,  according  to  Henning,  is 
the  determining  factor  so  far  as  odor  is  concerned. 

Henning  is  further  convinced  that  in  a  general  way 
types  of  chemical  constitution  can  be  indicated  for  the 
six  groups  of  odors  that  he  was  able  to  distinguish 
(Fig.  18).  Thus  the  class  of  spicy  odors  is  represented 
by  compounds  in  which  the  osmophoric  groups  are  in 
para-position  (Fig.  18a),  as  in  anisaldehyde.  In  the 
flowery  odors  the  osmophoric  groups  are  in  the  meta-  or 
the  ortho-positions  (Fig.  18b),  as  in  tuberon.  In  the 
fruity  odors  the  groups  are  forked  (Fig.  18c)  as  in  cit- 
ral.  In  the  resinous  odors  the  groups  are  within  the 
ring  (Fig.  18d)  as  in  pinene.  In  the  burnt  odors  the 
ring  is  smooth  (Fig.  18e)  as  in  pyridin,  and  in  the  foul 
odors  the  ring  is  fragmentary  (Fig.  18f )  as  in  cacodyl. 
In  this  way  each  class  of  odors  is  associated  with  a  spe- 
cial feature  in  the  constitution  of  the  molecule  though 
not  necessarily  with  a  particular  osmophoric  group.  In- 
termediate odors  are  due  to  combinations  of  groupings 
which  partake  of  the  nature  of  the  two  classes  between 
which  the  intermediate  lies.  Thus  vanillin  has  an  odor 
between  spicy  and  flowery  and  its  three  osmophoric 


groups  (Fig.  18g)  are  attached  so  as  to  represent  both 
the  para-position  (spicy)  and  the  ortho-position  (flow- 
ery). By  this  ingenious  system  Henning  has  attempted 
to  connect  odor  with  chemical  constitution  and  though 

Fio.  IS. — Generalized  diagrams  of  the  molecular  structure  of  the  six  classes  of  aromatic 
bodies  that  serve  as  olfactory  stimuli  according  to  Henning  (1910);  a,  for  spicy  odors;  b.  for 
flowery  odors;  c,  for  fruity  odors;  d,  for  resinous  odors;  e,  for  burnt  odors;  f,  for  foul  odors; 
and  g,  for  au  intermediate  odor  between  spicy  and  flowery. 

the  attempt  is  avowedly  fragmentary  and  may  be  open 
to  much  subsequent  modification,  it  gives  promise  of 
the  solution  of  a  problem  that  heretofore  has  been 
most  baffling. 

9.  Inadequate  and  Adequate  Stimuli.  Inadequate 
olfactory  stimuli  are  apparently  very  few  in  number  and 
not  well  known.  Thermal  stimuli  when  applied  to  the 


olfactory  organs  are  said  to  call  forth  no  sensations  of 
smell,  and  Valentin's  statement  that  mechanical  stimuli 
will  produce  unpleasant  olfactory  sensations  has  not  been 
confirmed.  Aronsohn  (1884b),  after  filling  the  nasal 
cavity  with  warm  physiological  salt  solution  led  a  direct 
electric  current  through  this  cavity  with  the  result  that 
certain  obscure  sensations  were  produced  depending  upon 
whether  the  anode  or  the  cathode  was  within  the  nose. 
With  the  anode  in  the  nose  a  sensation  was  called  forth 
on  opening  the  circuit;  with  the  cathode  in  the  nose  on 
closing  it.  There  was,  however,  no  evidence  to  show 
that  these  effects  were  not  due  to  a  stimuation  of  tri- 
geminal  endings  instead  of  olfactory  endings.  Althaus 
in  1869  recorded  as  the  outcome  of  electrical  stimulation 
a  phosphorous-like  smell  in  a  patient  suffering  from 
double  trigeminal  paralysis.  Apparently  the  electric 
current  is  a  true  inadequate  stimulus  for  the  olfactory 
organ,  but  its  peculiarities  are  very  incompletely  under- 
stood. Aside  from  this  and  the  effects  from  solutions 
as  described  by  Veress,  inadequate  olfactory  stimulation 
seems  not  to  exist. 

The  adequate  olfactory  stimulus  for  both  water-in- 
habiting and  air-inhabiting  vertebrates  is  a  solution  in 
contact  with  the  olfactory  hairs  and  perhaps  formed  in 
part  within  these  bodies.  The  solvent  is  probably  first 
the  olfactory  mucous  which  receives  the  solute  from  the 
current  of  water  or  of  air  that  passes  over  its  outer  sur- 
face. This  watery  solvent,  which  from  its  nature  must  be 
almost  universal  in  its  dissolving  power,  passes  the  solute 
on  to  the  olfactory  hairs  whose  capacity  as  receptors  is 
probably  limited  by  their  lipoid  composition.  Only  those 
substances  that  are  soluble  in  lipoids  can  be  taken  up  by 



the  hairs,  a  process  that  must  precede  the  initiation  of  the 
olfactory  nerve-impulse.  The  solute  may  be  any  one 
of  an  immense  variety  of  substances  whose  primary  char- 
acteristics are  that  they  are  not  only  soluble  in  water  but 
also  in  oil.  The  amount  of  these  substances  necessary 
for  olfaction  even  in  the  case  of  the  least  odorous  of 
them  is  very  small  and  in  that  of  the  most  odorous  in- 
credibly small.  The  amounts  that  are  usually  estimated 
for  olfaction  are  those  contained  in  what  is  believed  to 
be  the  minimum  volume  of  water  or  of  air  necessaiy 
for  stimulation,  but  of  the  very  minute  amount  of  odor- 
ous substance  contained  in  this  volume  only  a  veiy  small 
fraction  of  it  can  reach  the  olfactory  hairs.  Much  must 
be  carried  away  in  the  general  current  or  left  stranded 
on  non-olfactory  portions  of  the  nasal  surfaces.  Whether 
the  olfactory  hairs  can  concentrate  this  material  or  not 
remains  to  be  ascertained,  but  even  assuming  that  they 
can,  the  effective  concentration  must  be  of  an  extremely 
low  order. 

The  substances  thus  brought  in  solution  into  the  ol- 
factory hairs  must  initiate  those  nervous  changes  that 
eventually  produce  the  olfactory  sensations.  There 
ought,  therefore,  to  be  some  relation  between  these  sub- 
stances and  the  resulting  sensations.  It  is  generally 
assumed  that  the  substances  that  act  as  olfactory  stimuli 
fall  into  classes  associated  with  corresponding  classes 
of  sensations.  As  already  indicated  this  conception  may 
be  called  the  component  theory  of  olfaction,  and  if  we 
assume,  for  instance,  that  the  six  classes  of  odors  dis- 
tinguished by  Henning  are  separate  classes,  a  view  that 
Kenning,  however,  opposes,  then  these  classes  would 


represent  the  olfactory  components  that  physiologists 
have  sought  for  so  long. 

The  very  existence  of  partial  anosmia  implies  olfac- 
tory components  the  inactivity  of  one  of  which  is  ac- 
countable for  the  partial  defect.  But  such  cases  are  too 
little  known  to  admit  of  clear  interpretation.  Thus 
Aronsohn's  observation  (1886)  that  partial  anosmia  pro- 
duced by  the  exhaustion  of  the  nose  through  ammonium 
sulphide  leaves  that  organ  sensitive  to  etherial  oils  but 
insensitive  to  hydrogen  sulphide,  hydrochloric  acid  and 
bromine,  may  be  a  differential  effect  between  true  odors 
(olfactory  endings)  and  irritants  (trigeminal  endings), 
and  not  between  groups  of  true  odors.  Nevertheless  it 
must  be  in  this  direction  that  an  experimental  analysis  of 
the  general  problem  of  olfaction  will  eventually  proceed. 

From  this  standpoint  the  condition  presented  by 
mixed  odors  is  of  significance.  At  least  two  classes  of 
odor  mixtures  are  to  be  distinguished,  one  spurious  and 
the  other  real.  Spurious  mixed  odors  are  those  in  which 
the  gases  or  vapors  act  chemically  on  each  other  and  thus 
produce  a  third  substance  which  may  or  may  not  have 
an  odor  of  its  own.  Thus  ammonia  and  acetic  acid  both 
stimulate  the  nose,  but  when  mixed  they  possess  no  odor 
for  they  combine  to  form  odorless  ammonium  acetate. 
Obviously  such  instances  are  not,  accurately  speaking, 
instances  of  mixed  odors.  On  the  other  hand  there  are 
many  pairs  of  odorous  substances  in  which  one  member 
does  not  act  upon  the  other  chemically  and  consequently 
in  which  the  two  are  left  to  act  independently  on  the  ol- 
factory receptors.  Such  double  stimuli,  from  the  stand- 
point of  the  component  theory  might  be  expected  to  excite 
two  sensations,  but  apparently  this  is  not  always  the 


case.  If  in  a  pair  of  such  odors  one  is  much  stronger 
than  the  other,  its  smell  dominates  completely.  If,  how- 
ever, the  two  odors  are  closely  balanced  a  true  odor  may 
result  which  in  quality  is  said  to  be  unlike  that  of  either 
component.  Novel  odors  of  this  kind  may  be  produced, 
according  to  Aronsohn  (1886),  by  such  combinations  as 
cologne  water  and  oil  of  orange,  cologne  water  and  oil  of 
lemon,  oil  of  bergamot  and  oil  of  orange,  and  so  forth. 
The  condition  that  thus  produces  a  noval  odor  is  one  of 
considerable  delicacy  and  may  be  easily  upset  by  the 
greater  exhausting  effect  of  one  or  other  of  the  components 
thus  allowing  the  less  exhausted  member  to  assert  itself 
and  to  call  forth  its  own  peculiar  sensation.  The  presence 
of  a  sensation  different  from  those  of  the  pair  of  stimuli 
producing  that  sensation,  might  seem  to  be  a  condition 
adverse  to  the  component  theory,  but  it  must  be  remem- 
bered that  in  vision,  in  which  the  component  conception 
is  fundamental,  an  exact  parallel  occurs.  Thus  when  a 
pure  orange  light  is  mixed  with  a  pure  green  light,  there 
may  result  a  sensation  of  yellow  that  is  wholly  unlike 
that  appropriate  to  either  member  of  the  combination, 
and  that,  as  a  matter  of  fact,  may  be  indistinguishable 
from  a  sensation  of  yellow  produced  by  a  pure  yellow 
]  ight.  Thus  in  accepting  the  component  theory  of  sensory 
activity  it  must  be  admitted  that  two  stimuli  together 
may  excite  a  receptor  in  precisely  the  same  way  as  a 
third  and  entirely  different  stimulus  may  do.  The  exist- 
ence of  a  novel  olfactory  sensation  due  to  the  simultane- 
ous activity  of  two  independent  stimuli  is  therefore,  no 
serious  obstacle  to  this  theory. 

The   condition  of  double  olfactory  stimulation   that 
has  just  been  described  must  not  be  confused  with  a  kind 


of  double  stimulation  that  has  been  much  studied.  Val- 
entin observed  that  when  ether  and  balsam  of  Peru  were 
smelled  at  the  same  time  one  by  one  nostril  and  the  other 
by  the  other  nostril,  the  odors  are  perceived  not  together 
but  alternately  and  Valentin  believed  that  there  was 
a  sensory  conflict  here  as  in  vision,  when  one  eye  is 
directed  to  a  field  of  one  color  and  the  other  eye  to  one 
of  another  color.  Aronsohn  (1886)  noted  a  similar  con- 
flict between  the  smell  of  camphor  and  that  of  oil  of 
lemon.  He  also  discovered  that  under  similar  circum- 

Table  VI. 
Pairs   of   neutralizing   odors    (Zwaardemaker,    1895,   p.    168). 

Pairs  of  odorous  bodies  Neutralizing  Strength 

in  olfacties 

Cedarwood  and  rubber 2.75 : 14 

Benzoin  and  rubber 3.5    : 10 

Paraffin  and  rubber 8.5    : 14 

Rubber  and  wux 14      :28 

Rubber  and  balsam  of  Tolu 14      :70 

Wax  and  balsam  of  Tolu 40      :90 

Paraffin  and  wax 10      :20 

stances  one  smell  could  overcome  another.  Thus  the 
smell  of  camphor  was  neutralized  by  the  smell  of  pe- 
troleum, cologne  water,  oil  of  juniper  and  so  forth. 
This  question  was  investigated  much  more  fully  by 
Zwaardemaker  (1895)  who  employed  for  this  purpose 
his  double  olfactometor.  By  this  means  it  was  compara- 
tively easy  to-  balance  odors  and  then  lead  one  into  one 
nasal  cavity  and  the  other  into  the  other  cavity.  In  this 
way  complete  neutralization  could  be  attained  with  great 
accuracy.  Table  VI  gives  a  list  of  neutralizing  pairs  of 
odors  and  the  intensity  in  olfacties  at  which  Zwaarde- 
maker found  neutralization  to  occur. 


It  is  needless  to  say  that  since  in  this  form  of  double 
stimulation  one  stimulus  is  applied  to  one  olfactory  organ 
and  the  other  to  the  other  organ,  the  phenomenon  of  neu- 
tralization cannot  depend  upon  the  chemical  action  of 
one  odor  upon  the  other,  for  the  odorous  materials  are 
not  allowed  to  mingle.  The  fact  that  they  are  separately 
applied  to  different  receptors  shows  that  this  type  of 
conflict  and  of  neutralization  must  have  a  central  origin. 

10.  Olfactory  Eeflexes.  In  discussing  the  relations 
of  the  two  categories  of  nasal  stimuli,  irritants  and  true 
odors,  Frohlich  attributed  reflex  action  to  the  first  but 
not  to  the  second,  and  it  is  true  that  nasal  irritants 
almost  invariably  call  forth  vigorous  respiratory  re- 
sponses, such  as  sneezing,  whereas  true  odors  are  seldom 
followed  by  reactions  of  a  marked  kind.  Pawlow,  how- 
ever, has  pointed  out  the  great  importance  of  true  odors 
in  exciting  and,  in  a  way,  in  controlling  the  whole  chain 
of  digestive  secretions,  a  process  just  as  significantly 
reflex  as  sneezing  but  not  so  easily  observed.  Both 
classes  of  stimuli,  then,  are  followed  by  abundant  and 
important  reflexes,  but  in  one  class  these  are  of  a  kind 
easily  noticed,  in  the  other  they  are  more  hidden. 

Although  the  olfactory  organs  in  man  are  unques- 
tionably concerned  with  the  odors  of  the  food  that  is 
being  masticated,  they  are  much  more  concerned  with 
the  odors  of  the  environment.  From  this  standpoint  the 
olfactory  organs  are  properly  classed  as  distance-recep- 
tors or  receptors  affected  by  stimuli  that  emanate  from 
more  remote  points  in  the  surroundings.  In  consequence 
our  olfactory  sensations  are  in  a  way  projected  into  the 
exterior  and  we  seek,  avoid,  or  recognize  the  distant  body 
by  its  odor.  The  smell  of  a  skunk  is  unquestionably  a 


protective  odor  in  that  it  implies  that  it  can  be  sensed 
by  other  animals  that  will  thereupon  avoid  its  source. 
The  great  delicacy  of  olfaction  among  the  higher  animals 
by  which  they  can  scent  the  hunter  is  well  known.  Other 
odors  have  much  to  do  with  sexual  activities  whereby  one 
sex  is  led  to  find  the  other  or  is  otherwise  excited  to  ac- 
tivity. But  the  prime  service  of  olfaction  is  in  the  quest 
of  food.  From  the  fishes  to  the  mammals  olfaction 
serves  as  a  means  of  discovering  hidden  or  remote  food 
and  in  this  respect  it  is  a  highly  significant  sense  for  the 
direction  of  locomotion.  In  man  and  other  microsmatic 
forms  much  of  the  keenness  of  olfaction  has  disappeared 
and  yet  the  high  development  of  this  sense  in  our  an- 
cestry has  left  such  a  profound  impression  on  the 
organization  of  our  central  nervous  apparatus  that  we  are 
often  surprised  by  the  power  of  our  olfactory  associations. 


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Contents. — 1.  Vomero-nasal  Organ  in  Man.  2.  Com- 
parative Anatony.  3.  Histology.  4.  Adjacent  Parts. 
5.  Function.  6.  Bibliography. 

VOMERO-NASAL  Organ  in  Man.  In  early  infancy  all 
human  beings  show  traces  of  a  pair  of  organs  that  are 
without  doubt  homologues  of  the  vomero-nasal  organs  of 
the  lower  vertebrates.  Each  vomero-nasal  organ  in  the 
new-bora  babe  is  a  short  tubular  structure  from  a  half 
to  two  and  a  half  millimeters  long  and  lodged  in  the  lower 
anterior  portion  of  the  nasal  septum.  The  organ  opens 
into  the  nasal  cavity  by  a  minute  pore  on  the  free  sur- 
face of  the  septum  not  far  from  its  ventral  border  and 
onlv  a  short  distance  inward  from  the  external  naris. 


The  tubular  part  of  the  organ  extends  posteriorly  from 
this  minute  pore  and  ends  blindly  at  a  point  somewhat 
higher  than  the  level  of  the  pore  itself. 

In  early  human  embryos  the  pore  of  the  vomero-nasal 
organ  can  be  easily  identified  on  the  median  face  of  the 
nasal  chamber  just  within  the  anterior  naris  (Fig.  19).  In 
adults  the  organ,  though  commonly  present,  may  disap- 
pear completely.  When  present  it  occurs  near  the  ven- 
tral margin  of  the  nasal  septum  (Fig.  20).  Kolliker 
(1877)  states  that  it  may  vary  in  length  from  two  to 
seven  millimeters  and  Anton  (1895)  gives  as  the  extremes 
2.2  millimeters  and  8.4  millimeters.  As  seen  in  trans- 
verse section  it  has  the  appearance  of  a  tube  flattened  in 
the  plane  of  the  nasal  septum.  Its  lateral  wall  is  cov- 




ered  with  an  epithelium  that  resembles  histologically  the 
respiratory  epithelium  of  the  nasal  cavity.  This  lateral 
epithelium  may  even  be  ciliated.  The  median  wall  is  cov- 
ered with  an  epithelium  much  like  the  olfactory  epithelium 
of  the  nose  except  that  differentiated  olfactory  cells  are 
apparently  not  present.  The  cavity  of  the  organ  is 
sometimes  obliterated  by  excessive 
epithelial  growth  and  calcareous  con- 
cretions may  occur  in  its  walls.  As  it 
appears  to  be  without  nervous  con- 
nections, the  vomero-nasal  organ  in 
man  is  probably  entirely  rudimentary. 
2.  Comparative  Anatomy.  A  vo- 
mero-nasal organ  has  been  recog- 
nized for  some  time  past  in  all  classes 
of  vertebrates  except  the  fishes,  but, 
according  to  Gawrilenko  (1910),  this 
group  too  must  be  admitted  to  have  at 
least  the  f  oreshadowings  of  such  an 


as  the  sharks  and  rays  each  oltactory 
sac  is  divided  into  two  compartments 
with  separate  innervation  and  these  two  compartments 
may  be  supposed  to  correspond  one  to  the  vomero-nasal 
organ  and  the  other  to  the  olfactory  organ  proper.  This 
double  character  of  the  olfactory  apparatus  is  also  seen  in 
other  fishes.  Thus  in  the  development  of  the  olfactory  sac 
of  the  salmon  Gawrilenko  has  shown  that  this  organ 
includes  two  sensory  thickenings  or  placodes,  a  median 
one  and  a  lateral  one.  These  two  placodes  can  be 
traced  into  the  adult  where  they  are  said  to  give  rise  to 
a  median  olfactory  area  and  a  lateral  area.  The  median 

FIG.  19.—  Lateral  view 

-.-.  .  •,  ...  ^  of  the  head  of    a  human 

Even  in  such  primitive  iorms    embryo  showing  the  pore 

(v)   of    the    vomero-nasal 

After  HIS,  isss, 



area  is  believed  to  correspond  to  the  vomero-nasal  organ 
of  the  higher  vertebrates  and  the  lateral  area  to  the  true 
olfactory  receptor  of  these  forms. 

In  some  amphibians  the  distinction  between  a  lateral 
and  a  median  organ  is  much  more  evident  than  in  fishes 
(Fig.  21).  The  lateral  organ  is  the  one  that  conducts  the 

Fio.  20. — Diagram  of  the  median  face  of  the  left  nasal  ravity  of  man ;  the  small  circle  marks 
the  position  of  the  vomero-nasal  organ  in  the  nasal  septum. 

newly  established  air  current  from  the  external  naris  to 
the  choana  and  hence  corresponds  to  the  olfactory  organ 
proper.  The  median  cavity  is  less  involved  in  this  cur- 
rent and  is  believed  to  represent  the  vomero-nasal  organ. 
In  certain  sauropsida  such  as  the  alligators  and  tur- 
tles the  vomero-nasal  organ  has  been  said  to  be  at  best 
only  poorly  developed,  though  so  far  as  turtles  are  con- 
cerned this  opinion  is  not  shared  by  one  of  the  most 
recent  workers,  McCotter  (1917).  In  birds  the  organ 
is  claimed  to  be  entirely  absent,  but  in  lizards  and  in 
snakes  it  is  highly  differentiated  (Fig.  22).  Here  the 
olfactory  apparatus  consists  of  a  well-developed  organ 



of  smell  located  in  the  respiratory  passage  and  an 
entirely  independent  vomero-nasal  organ.  The  latter, 
in  the  form  of  a  blind  sac,  opens  into  the  cavity  of  the 
mouth.  This  peculiarity  is  probably  dependent  upon  the 
growth  of  the  hard  palate  in  reptiles  whereby  a  new 
adjustment  between  the  nasal  cavity  and  the  mouth  is 
brought  about. 

In  mammals  the  vomero-nasal  organ  also  shows  much 
diversity.    It  is  apparently  best  developed  in  the  lower 

Fia.  21. — Transverse  section  of  the  snout  of  a  young  frog  snowing  the  partial  division 
of  the  nasal  cavity  into  a  lateral  or  olfactory  portion   (o)  and 
a  median  or  vomero-nasal  portion  (v). 


forms,  such  as  the  Australian  duckbill  Ornithorhynchus, 
and  it  is  rudimentary  in  such  groups  as  the  primates 
including  man.  In  general  it  has  the  form  of  a  blind 
sac  that  opens  usually  by  means  of  the  naso-palatine  duct 
(Stenson's  duct)  into  the  mouth,  a  relation  that  is  prob- 
ably reminiscent  of  its  original  connection  with  the 
primitive  choana  of  which  the  naso-palatine  duct  may 
be  regarded  as  a  trace.  Less  commonly  it  opens  directly 
by  its  own  duct  into  the  nasal  cavity.  This  condition 
obtains  in  certain  rodents  such  as  the  rabbit,  guinea  pig, 
rat,  and  mouse,  and  in  certain  primates  including  man. 



In  all  these  higher  vertebrates  the  olfactory  organ  proper 
corresponds  to  the  lateral  component  of  the  pair  of 
organs  in  the  lower  forms  and  the  vomero-nasal  organ 
to  the  median  member  of  this  group  (Fig.  23). 

3.  Histology.  The  vomero-nasal  organ  of  the  dog 
and  the  cat,  as  described  by  Bead  (1908),  is  a  tubular 

organ  whose  transverse 
section  is  circular  in  out- 
line near  its  opening 
and  crescentic  or  kidney- 
shaped  throughout  its 
greater  extent.  Its  median 
wall  may  be  two  to  three 
times  as  thick  as  its  lateral 
wall.  This  thickened  por- 
tion, which  has  been 
observed  by  numerous 
workers  in  a  variety  of 
mammals,  is  similar  in 

Fin.  22.— Transverae  section  of  the  head  Cellular  Composition  to  the 

of    a  snake  embryo  (Agkistrodon)    showing  ,   .                                 -JIT                    c 

the    nasal    canal   proper     (n)   and    the    large  OltaCtOry        epithelium        Ol 

vomero-nasal     (v)     organ     opening     on     the  '    . 

roof   of   the     mouth.     Preparation    by    Mr.  tll6    11OSC    aild    IS    111    StrOllg 
F.  B.    Manning. 

contrast  with  the  lateral 

thin  wall  which  resembles  respiratory  nasal  epithelium. 
Read  has  shown  that  the  vomero-nasal  organ  of  the  cat 
and  the  dog  is  like  the  olfactory  region  of  the  nose  in  that 
it  receives  nerve  fibers  from  two  sources,  the  olfactory 
nerve  and  the  trigeminal  nerve. 

As  early  as  1892  von  Brunn  showed  that  the  sense 
cells  of  the  vomero-nasal  organ  of  the  sheep  were  con- 
nected with  nerve  fibers  in  exactly  the  way  they  were  in 
the  olfactory  region  proper  and  he  assumed,  probably 



with  correctness,  that  these  fibers  belonged  to  the  olfac- 
tory nerve  (Fig.  24).  These  observations  were  confirmed 
by  all  subsequent  workers  including  von  Lenhossek  (1892) 
in  the  rabbit,  Retzius  (1894)  in  the  snake,  Ramon  y  Cajal 
(1895)    in    the    rat,    and    Read    (1908)    in    the   kitten. 
Retzius  showed  that  in  the  snake  those  nerve-fibers  that 
were   connected   with   the   sense   cells   in   the   vomero- 
nasal  organ  mingled  with  the  bundle  of  fibers  from  the 
olfactory    region    of 
the  nose  and  thus  con- 
firmed  von   Brunn's 
suspicion     that     vo- 
true  olfactory  fibers. 

Von  Lenhossek 
pointed  out  that  at 
least  in  the  rabbit  the 
sense  cells  were  not 
limited  to  the  thick- 
;ened  face  of  the  vo- 
inero-nasal  organ,  as 
had  been  maintained  heretofore,  but  were  found  upon  the 
opposite  tlu'n  face  of  the  organ  as  well  This  observation 
was  confirmed  on  the  rat  a  few  years  later  by  Ramon  y 
Cajal.  Hairlike  terminations  on  the  vomero-nasal  sense 
cells,  such  as  those  that  had  been  found  in  the  olfactory 
cells,  were  sought  for  by  a  number  of  investigators 
but  only  traces  of  these  structures  could  be  found  (von 
Brunn,  Retzius,  Read),  probably  because  of  the  ease 
with  which  they  are  Destroyed  in  the  preparation  of 
the  tissue. 

Von    Lenhossek    in    1892    not    only    confirmed    von 


FIG.  23. — Transverse  section  of  the  nasal 
septum  of  a  young  cat  showing  the  vomero- 
nasal  organ  (v),  its.cartilage  (c),  and  the  nasal 
cavity  (n). 



Brunn's  observation  that  the  vomero-nasal  sense  cells 
were  directly  connected  with  nerve  fibers,  but  he  also 
pointed  out  that  in  the  Jacobson  organ  of  the  foetal  rab- 
bit free-nerve  terminations  occurred.  These  free  termi- 
nals in  some  instances  reached  the  receptive  surface  of 
the  epithelium  where  they  ended  in  slight  knobs.  Similar 

endings  were  recorded  for  the 
rat  by  Bamon  y  Cajal  (1895). 
Von  Lenhossek  was  unable  to 
decide  definitely  whether  these 
terminals  belonged  to  the  olfac- 
tory or  to  the  trigeminal  nerve. 
Nor  is  this  question  definitely 
settled  now,  though,  judging 
from  the  conditions  met  with 
in  the  olfactory  organ  of 
the  nose,  it  is  highly  prob- 
able, as  Eead  concludes,  that  they  belong  to  the  trigem- 
inal nerve.  Admitting  this  to  be  the  case,  the  innervation 
of  the  vomero-nasal  organ  would  agree  in  all  particulars 
with  that  of  the  olfactory  organ  proper.  It  is  quite 
clear  from  the  studies  of  Brookover  (1917)  on  the  ner- 
vus  termmalis  as  well  as  from  those  of  Larsell  (1918) 
that  the  relations  of  this  nerve  to  the  vomero-nasal  organ 
are  merely  incidental;  the  terminal  nerve  is  in  no  sense 
especially  connected  with  the  organ  of  Jacobson. 

4.  Adjacent  Parts.  In  many  of  the  higher  verte- 
brates the  vomero-nasal  organ  is  contained  within  a  more 
or  less  complete  capsule  of  cartilage,  the  Jacobson  car- 
tilage (See  Fig.  23).  In  the  cat  this  capsule,  according 
to  Read,  is  complete  anteriorly  and  incomplete  posteri- 
orly; in  the  dog  it  is  incomplete  throughout  its  whole 

Fio.  24. — Epithelium  from  the 
vomero-nasal  organ  of  the  sheep 
showing  the  receptive  cells  impreg- 
nated by  the  Golgi  method.  After 
von  Brunn,  1892,  Plate  30,  Fig.  12. 


length.  The  vomero-nasal  organ  of  these  forms  has  com- 
monly associated  with  it  a  considerable  amount  of  caver- 
nous tissue.  This  tissue,  which  was  long  ago  identified 
in  nasal  organs  by  Klein  (1881a,  1881b),  is  so  disposed 
that  in  connection  with  the  surrounding  cartillage  and 
other  parts,  it  may  serve  as  a  means  of  changing  in  no 
small  degree  the  volume  of  the  organ. 

5.     Function.    Concerning  the  function  of  the  vomero- 
nasal  organ  almost  nothing  is  known.     Von  Mihalkovics 
(1898)   found  that  after  burning  out  the  naso-palatine 
duct    and    more    or    less    of    the    vomero-nasal    organ 
in  the  cat  and  in  the  rabbit,  the  appropriation  of  food 
by  these  animals  was  not  interfered  with,  but  it  is  hardly 
to  be  expected  that  so  crude  an  experiment  as  this  would 
yield  significant  results.     Kolliker  emphasized  the  fact 
that,  at  least  in  mammals,  the  connection  between  the 
vomero-nasal  organ  and  the  exterior  is  so  narrow  and 
indirect  that  it  seems  almost  impossible  that  there  should 
be  any  transfer  of  material  from  the  exterior  to  the  inte- 
rior of  the  organ  as,  for  instance,  is  implied  in  olfaction. 
He,  therefore,  suggested  that  the  vomero-nasal  organ 
was  concerned  with  testing  the  animal's  own  juices  as  rep- 
resented by  the  secretions  from  this  organ.    But  the 
vomero-nasal  organ,  particularly  in  mammals,  is  inti- 
mately  associated   with   much   cavernous   tissue   whose 
change  in  volume  may  be  concerned  with  its  filling  and 
emptying.     Thus  it  is  quite  possible  that  oral  or  nasal 
juices  may  be  sucked  into  the  vomero-nasal  organ  and 
discharged  from  it  as  has  recently  been  maintained  by 
Broman  (1918).     Henning  (1916)  has  suggested  that  the 
organ  is  concerned  with  water  olfaction  as  contrasted 
with  air  olfaction,  but  according  to  an  unpublished  obser- 


vation  of  Mr.  H.  E.  Hamlin  air  is  often  found  in  the 
vomero-nasal  organs  of  freshly  killed  mammals,  and  this 
observation  when  taken  in  connection  with  the  work  of 
Broman  supports  the  hypothesis  already  advanced  by 
many  investigators  (P.  and  F.  Sarasin,  1890;  Seydel, 
1895;  Gaupp,  1900)  that  these  organs  are  subsidiary 
olfactory  receptors,  an  opinion  that,  while  it  lacks  com- 
plete experimental  proof,  is  abundantly  supported  by  the 
finer  structure  of  the  parts  concerned. 

ANTON,  W.   1895.     Beitriige  zur  Kenntnis  dos  Ja^obson't-chen  Organes  bei 

Erwachsenen.    Zeitschr.  Eeitk.,  Bd.  16,  pp.  355-372. 
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Contents. — 1.  Common  Chemical  Sense  in  Man.  2.  In 
Lower  Vertebrates.  3.  Nerve  Terminals.  4.  Relation 
to  Other  Senses.  5.  Bibliography. 

1.  COMMON  Chemical  Sense  in  Man.  It  was  long1 
ago  made  clear  by  Frohlich  that  on  the  nasal  surfaces 
of  man  there  were  two  systems  of  receptors  that  could 
be  stimulated  by  gaseous  or  vaporous  materials:  olfac- 
tory cells  representing  the  olfactory  nerve,  whose  stim- 
uli, delicate  perfumes  and  odors,  call  forth  few  observable 
responses,  and  free-nerve  terminals  probably  represent- 
ing the  trigeminal  nerve,  whose  stimuli,  irritants  for  the 
most  part,  are  usually  followed  by  vigorous  reactions 
such  as  sneezing.  This  distinction  has  been  generally 
accepted  among  physiologists,  but  it  has  not  been  so 
clearly  seen  that  the  receptors  for  irritants  are  found  in 
other  parts  of  the  body  than  the  nose  and  that  they  rep- 
resent a  fairly  well  denned  category  of  sense  organs 
which,  if  not  so  sharply  marked  off  as  those  of  taste  and 
of  smell,  are  fairly  comparable  in  distinctness  with  the 
receptors  for  heat,  cold,  or  pain.  The  extent  of  their 
occurrence  is  easily  recognized.  Thus  the  vapor  of 
ammonia  not  only  irritates  the  nose,  but  also  the  eye, 
causing  watering,  as  well  as  the  mouth  and  the  upper 
respiratory  region  whence  arise  impulses  that  lead  to 
coughing  and  choking.  Irnlnnts  of  this  kind  also  stim- 
ulate the  anus  and  the  genital  apertures  and  in  fact  any 



part  of  the  body  where  a  mucous  surface  is  in  contact 
more  or  less  with  the  exterior.  In  man,  then,  the  recep- 
tors for  irritants  have  a  much  wider  distribution  over 
the  body  than  the  olfactory  receptors  have  in  that  they 
are  found  on  almost  every  exposed  or  partly  exposed 
mucous  surface. 

2.  In  Lower  Vertebrates.  In  other  mammals  than 
man,  in  birds,  and  in  reptiles  the  receptors  for  irritating 
substances  are  probably  distributed  in  much  the  same 
way  as  in  man  and  are  confined  to  the  exposed  or  semi- 
exposed  mucous  surfaces.  In  the  amphibians  and  the 
fishes,  however,  this  system  of  receptors  shows  a  pro- 
digious expansion  in  that  in  these  animals  it  is  found 
covering  their  whole  exteriors.  The  well  known  experi- 
ment of  stimulating  the  frog's  foot  with  solutions  of 
acids  and  other  such  substances  is  based  upon  this  peculi- 
arity and  the  sensitiveness  of  the  skin  of  this  and  other 
amphibians  and  of  fishes  as  worked  out  by  Nagel  (1894), 
Parker  (1908a,  1908b,  1912),  Sheldon  (1909),  Cole  (1910), 
Crozier  (1915,  1916),  and  others  show  quite  clearly  that 
sensitiveness  to  solutions  of  chemicals  is  a  common 
property  of  the  skin  in  all  these  aquatic  vertebrates. 

As  early  as  1894  Nagel  discovered  that  the  integument 
of  the  dogfish  Scyllium  was  extremely  sensitive  to  a  great 
variety  of  chemical  substances.  He  likewise  found  that  the 
skin  of  the  goosefish  Lophius  and  of  the  lancetfish  Amphi- 
oxus  were  also  generally  open  to  chemical  stimulation. 

Nagel 's  observations  on  Amphioxus  were  confirmed 
in  1908  when  it  was  shown  that  the  skin  of  this  fish  was 
sensitive  to  solutions  of  acids,  alkalis,  alcohol,  ether, 
chloroform,  turpentine,  oil  of  bergamot  and  oil  of  rose- 
mary but  not  to  solutions  of  sugar.  It  was  also  demon- 


strated  that  the  skin  of  the  catfish  Amiurus  was  sensitive 
to  sour,  saline,  and  alkaline  solutions,  a  condition  that 
was  subsequently  found  to  be  true  for  the  young  of  the 
lamprey  eel  Ammocoetes  Parker  (1908b,  1912).  In  1909 
Sheldon  published  an  account  of  the  chemical  stimulation 
of  the  skin  of  the  dogfish  Mustelus,  the  most  extensive 
study  of  this  kind  thus  far  made.  Sheldon  found  that 
the  whole  outer  surface  of  this  fish  was  very  sensitive 
to  acids  and  alkalis,  less  so  to  salts  and  bitter  substances 
and  not  at  all  to  sugar  solutions,  a  condition  that  in  gen- 
eral confirmed  the  results  of  earlier  workers.  Crozier 
(1915)  studied  the  mutual  relations  of  salts  of  sodium, 
potassium,  and  calcium  as  applied  to  the  frog's  skin  and 
was  able  to  demonstrate  ionic  antagonism  which  led  him 
to  conclude  that  in  normal  stimulation  the  surface  of  the 
receptor  must  be  penetrated  by  the  stimulant. 

These  observations  warrant  the  general  conclusion 
that  the  outer  surfaces  of  most  fishes  and  amphibians  are 
open  to  stimulation  by  chemical  substances  of  a  mildly 
irritating  kind.  It  is  probable  that  this  capacity  has 
been  retained  by  the  air-inhabiting  vertebrates  in  only 
a  very  circumscribed  and  local  way,  namely  on  those 
exposed  or  partly  exposed  mucous  surfaces  that  reproduce 
in  their  delicacy  and  moistness  the  characteristics  of  the 
general  outer  surface  of  aquatic  forms.  From  this 
standpoint  the  restriction  of  the  chemical  sensibility  of 
the  air-inhabiting  vertebrates  is  the  result  of  the  drying 
of  their  skins  in  consequence  of  an  ancestral  migration 
from  an  environment  of  water  to  one  of  air. 

3.  Nerve  Terminals.  The  form  of  nerve  terminal 
that  is  concerned  with  the  reception  of  chemical  irritants 
in  the  skin  of  vertebrates  is  well  indicated  in  the  catfish 


Amiurus.  If  a  bait  in  the  form  of  a  piece  of  meat  or  the 
like  is  held  close  to  the  flank  of  one  of  these  fishes,  the 
animal  is  very  likely  to  turn  suddenly  and  snap  it  up.  This 
is  not  a  surprising  response,  for  the  sides  of  these  ani- 
mals are  well  provided  with  taste-buds.  They  are  also 
supplied  with  lateral-line  organs.  Both  these  sets  of 
receptors  may  be  eliminated  by  cutting  on  the  one  hand, 
the  branch  of  the  facial  nerve  that  is  supplied  to  the  taste- 
buds  of  the  side  of  the  body  and,  on  the  other,  the  lateral- 
line  nerve  that  is  distributed  to  the  lateral-line  organs 
of  the  same  region.  After  the  fish  has  recovered  from 
such  an  operation,  it  will  no  longer  respond  to  a  bait  held 
near  its  flank,  but  the  skin  of  this  region  is  still  per- 
fectly open  to  stimulation  by  sour,  saline  and  alkaline 
solutions.  As  the  only  receptors  left  after  the  operation 
just  described  are  the  free-nerve  terminals  of  the  spinal 
nerves,  these  terminals  must  be  the  receptors  for  chem- 
ical irritants.  This  conclusion  is  in  accord  with  the  fact 
that  this  type  of  ending  is  the  only  one  that  occurs  in 
many  portions  of  the  skin  of  the  dogfish,  of  the  foot  of 
the  frog,  and  of  the  partly  exposed  mucous  surfaces  of 
the  higher  vertebrates  such  as  those  of  the  mouth  and 
the  nose.  Moreover  when  these  endings  are  rendered 
inoperative  by  cutting  their  nerve  trunks,  as  Sheldon  did 
on  the  dogfish  and  as  has  often  been  done  on  the  nasal 
cavities  of  mammals,  irritating  substances  are  no  longer 
effective.  Free-nerve  endings  of  spinal  or  cranial  nerves 
are,  therefore,  quite  certainly  the  type  of  nerve-terminal 
concerned  with  the  reception  of  chemical  irritants. 

4.  Relation  to  Other  Senses.  In  discussing  the  relation 
of  the  receptors  for  chemical  irritants  to  other  sense 
organs  some  of  the  earlier  workers  suggested  a  compari- 


son  of  these  receptors  with  those  for  taste  Parker 
(1908a);  Herrick  (1908).  More  recently  Cogliill  (1914) 
has  declared  that  since  tactile  and  chemical  irritability 
develop  simultaneously  in  certain  amphibian  larvae, 
chemical  irritability  is  in  reality  tactile  in  nature.  It 
must  also  be  perfectly  evident  that  the  receptors  under 
consideration  have  striking  resemblances  to  those  con- 
cerned with  pain. 

The  fact  that  organs  of  taste  always  involve  special- 
ized end-organs,  such  as  taste-buds,  whereas  receptive 
surfaces  for  chemical  irritants  may  contain  only  free- 
nerve  endings,  shows  that  the  relation  of  these  two 
classes  of  receptors  is  at  best  only  distant.  This  con- 
clusion is  supported  by  an  observation  by  Parker  and 
Stabler  (1913)  that  the  minimum  concentration  of  ethyl 
alcohol  necessary  for  the  stimulation  of  the  irritant 
receptors  in  man,  5  to  10  molar,  is  decidedly  stronger 
than  that  which  will  stimulate  the  human  gustatory 
organs,  3  molar. 

The  relation  of  the  receptors  for  irritants  to  those 
for  touch  and  for  pain  seems  to  be  clearly  indicated  in 
the  results  of  experiments  in  which  exhaustion  and  nar- 
cotics have  been  used.  If  the  tail  of  an  amphioxus  is 
subjected  to  about  twenty  applications  of  weak  nitric 
acid,  0.025  molar,  in  fairly  rapid  succession,  the  fish  will 
cease  to  respond  to  this  kind  of  stimulus.  After  the 
exhaustion  of  the  mechanism  for  this  type  of  reception, 
the  tail  of  the  fish  will  be  found  fully  sensitive  to  the 
touch  of  a  camel's  hair  brush.  If,  now,  the  tail  of 
a  fresh  individual  is  vigorously  stroked  some  thirty  times 
in  succession,  the  fish  will  cense  to  respond  to  this  form 
of  mechanical  stimulation  but  it  will  still  be  found  very 


sensitive  in  the  exhausted  part  of  the  skin  to  weak  acid. 
Thus  mechanical  stimulation  and  chemical  stimulation 
seem  to  apply  to  different  sets  of  terminals  and  the 
exhaustion  of  one  set  does  not  involve  that  of  the  other. 

On  treating  a  portion  of  the  surface  of  a  dogfish  with 
2  per  cent  cocaine,  Sheldon  found  that  tactile  stimulation 
ceased  in  from  ten  to  twenty  minutes  whereas  chemical 
stimulation  was  effective  for  a  somewhat  longer  period. 
By  continuing  the  treatment  with  cocaine  receptivity  for 
chemical  irritants  was  also  eventually  abolished.  In  a 
similar  way  Cole  (1910)  found  that  if  the  hind  foot  of  a 
spinal  frog  was  treated  with  1  per  cent  cocaine  till  the 
animal  no  longer  responded  to  pricking  or  scratching 
with  a  needle  or  to  pinching  with  forceps,  it  would  never- 
theless respond  vigorously  to  a  salt  solution.  The'se 
results  were  confirmed  by  Crozier  in  1916  who  used  a 
half  per  cent  solution  of  cocaine  hydrochloride  on  a 
frog's  foot.  After  about  20  minutes'  immersion  in  this 
solution,  the  reaction  time  of  the  cocained  foot  to  formic 
acid  0.05  molar,  was  about  twice  that  of  the  normal  foot. 
After  about  an  hour  to  an  hour  and  a  half  of  this  treat- 
ment the  cocained  foot  no  longer  reacted  to  pinching  but 
gave  good  responses  to  acid  with  reaction  times  of  from 
ten  to  fifteen  seconds,  about  twice  that  of  the  non-cocained 
foot.  These  observations  show  beyond  a  doubt  that  the 
effect  of  chemical  irritants  on  the  naturally  moist  skin 
of  vertebrates  is  not  to  be  ascribed  to  the  stimulation 
of  organs  of  touch  or  of  pain  but  to  some  other  form  of 
receptor,  the  terminals  of  what  has  been  called  the  com- 
mon chemical  sense. 

As  Crozier  has  pointed  out,  there  can  be  no  question 
of  the  distinctness  of  the  human  sensations  attributed 


to  the  common  chemical  sense  as  contrasted  with  our 
sensations  of  smell,  taste,  touch,  or  pain.  The  curious 
feeling  that  comes  from  vapors  that  irritate  the  eyes, 
nose,  or  even  the  mouth  has  not  the  remotest  relation 
to  touch,  smell,  or  taste  and  is  only  distantly  suggestive 
of  pain.  Pain,  however,  is  easily  separated  from  the 
common  chemical  sense  by  the  use  of  cocaine,  and  we 
are,  therefore,  entirely  justified  in  concluding  that  the 
common  chemical  sense  is  a  true  sense  with  an  indepen- 
dent set  of  receptors  and  a  sensation  quality  entirely  its 
own.  In  the  fishes  and  amphibians  it  pervades  the  whole 
integument  but  in  the  reptiles,  birds  and  mammals  it  is 
restricted  to  the  partly  exposed  mucous  membranes  of  the 
natural  apertures,  a  restriction  that  doubtless  arose  as 
the  vertebrate  changed  from  an  aquatic  to  an  air-inhabit- 
ing form. 


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Contents. — 1.  Distribution  of  Taste-buds  in  the  Oral 
Cavity  of  Man.  2.  Comparative  Distribution  of  Taste- 
buds.  3.  General  Form  of  Taste-buds.  4.  Cellular  Com- 
position of  Taste-buds.  5.  Intragemmal  and  Other 
Spaces.  6.  Innervation  of  Taste-buds.  7.  Gustatory 
Nerves.  8.  Eelation  of  Gustatory  Nerve  Fibers  and 
Taste-buds.  9.  Bibliography. 

1.  DISTRIBUTION  of  Taste-buds  in  the  Oral  Cavity  of 
Man.  In  man  the  organs  of  taste  are  located  in  the 
mouth.  These  are  the  so-called  taste-buds  discovered 
independently  by  Loven  (1867)  and  by  Schwalbe  (1867). 
In  the  adult  human  being  they  have  been  identified  on 
the  dorsal  surface  of  the  tongue  except  the  mid-dorsal 
region,  on  both  the  anterior  and  posterior  surfaces  of  the 
epiglottis,  on  the  inner  surface  of  the  arytenoid  process 
of  the  larynx,  on  the  soft  palate  above  the  uvula,  on  the 
anterior  pillars  of  the  fauces,  and  on  the  posterior  wall 
of  the  pharynx.  All  other  oral  surface  in  the  adult,  such 
as  the  lips,  the  gums,  the  cheeks,  the  inferior  surface  of 
the  tongue,  the  hard  palate,  the  uvula,  and  the  tonsils 
are  devoid  of  these  organs. 

In  young  individuals,  babes,  and  human  embryos 
taste-buds  are  more  widely  distributed  than  they  are  in 
the  adult.  According  to  Tuckerman  (1890a,  1890b)  and 
Graberg  (1898)  taste-buds  appear  in  man  at  about  the 
beginning  of  the  third  month  of  foetal  life.  Stahr  (1902) 
found  them  in  human  embryos  in  the  middle  of  the  dor- 


sum  of  the  tongue  and  Ponzo  (1905)  identified  them  on 
the  palatine  tonsils,  the  hard  palate,  and  the  cervical 
part  of  the  esophagus,  regions  from  which  they  are 
absent  in  the  adult.  As  early  as  1875  Hoffmann  called 
attention  to  the  fact  that  in  human  embryos  and  newly 
born  babes  taste-buds  were  commonly  found  on  the  free 
surfaces  of  the  vallate  papillae,  situations  from  which 
they  disappear  in  later  life.  This  observation  was  con- 
firmed by  Tuckerman  (1889)  as  well  as  by  Hermann 
(1885),  who,  however,  worked  upon  the  rabbit.  Thus  the 
gustatory  apparatus  of  man  and  of  other  mammals  is 
by  no  means  constant,  but  suffers  reduction  from  the  late 
embryonic  period  to  the  adult  state.  On  the  tongue  of 
man  the  reduction  is  chiefly  in  the  middle  region  of  the 
distal  two-thirds  so  that,  as  Stahr  (1902)  has  pointed 
out,  the  center  of  taste  in  this  organ  shifts  with  growth 
from  a  position  near  the  tip  of  the  tongue  to  one  in  the 
neighborhood  of  the  vallate  papillae.  This  opinion  is  in 
agreement  with  the  observation  of  Heiderich  (1906)  that 
after  birth  the  taste-buds  of  the  vallate  papillae  show 
almost  no  change. 

Wherever  taste-buds  occur  in  man,  except  on  the 
tongue,  they  are  found  simply  imbedded  in  the  epithe- 
lium of  the  mucous  membrane  of  the  region  concerned. 
On  the  tongue,  however,  they  are  almost  invariably  asso- 
ciated with  certain  kinds  of  papillae.  The  human  tongue 
possesses  several  classes  of  these  structures,  which  from 
their  forms  have  been  designated  as  conical,  filiform, 
fungiform,  foliate,  and  vallate.  The  plush  surface  of  the 
dorsum  of  the  tongue  is  produced  by  innumerable  fine 
conical  and  filiform  papillae.  These,  however,  almost 
never  have  taste-buds  associated  with  them.  The  other 



types  of  papillae,  the  fungiform,  foliate,  and  vallate,  very 
generally  carry  taste-buds  (Fig.  25). 

The  fungiform  papillae  are  relatively  large  knob-like 
elevations  scattered  over  the  dorsum  of  the  tongue. 
They  can  be  easily  seen  with  the  unaided  eye  and  may 
be  readily  located  and  identified.  They  commonly  carry 

a  few  taste-buds  embed- 
ded in  the  epithelium  of 
their  free  outer  sur- 
faces. In  sections  of  the 
crowns  of  these  papillae 
parallel  to  the  surface 
of  the  tongue  three  or 
four  or  more,  rarely  six 
to  eight,  taste-buds  may 
be  identified.  In  verti- 
cal section  it  can  be 
seen  that  the  taste-buds 
are  not  indiscriminately 
scattered  over  the  free 
surface  of  the  papilla, 
but  are  perched  on  the 
secondary  dermal  pa- 
pillae contained  within  the  papilla  proper  and  that  they 
always  reach  through  the  full  thickness  of  the  epidermis 
from  the  dermal  core  of  the  secondary  papilla  to  the  free 
outer  surface  of  the  primary  papilla  itself  (Fig.  26). 
This  extension  through  the  whole  thickness  of  the  epi- 
dermis seems  to  be  a  common  characteristic  of  taste- 
buds,  for  it  is  to  be  noted  in  them  from  fishes  to  man. 
It  is  an  easy  means  of  distinguishing  them  from  other 
bud-like  receptors  such  as  the  lateral-line  organs  whose 

Fio.  25. — Dorsal  view  of  the  human  tongue 
ehowing  foliate  papillw  (f)  and  vallate  papilla  (v). 


cells  extend  only  part  way  through  the  epithelium  in 
which  they  are  imbedded. 

The  foliate  papillae  lie  on  either  side  of  the  edge  of 
the  human  tongue  and  close  to  its  root.  They  form  a 
series  of  from  three  to  eight  vertical  parallel  ridges. 
Each  ridge  is  abundantly  supplied  with  taste-buds  which, 

Fio.  26. — Vertical  section  of  a  fungiform  papilla  showing  two  taste-buda. 

however,  do  not  occur  on  its  free  outer  surface  but  on 
its  sides.  Here  the  buds  open  into  the  ditch  between  the 
ridge  on  which  they  are  located  and  the  next  one.  In 
sections  transverse  to  the  axis  of  the  ridge  the  numbers 
of  taste-buds  seen  on  the  two  sides  of  a  given  ditch  may 
vary  from  three  to  twenty.  In  the  rabbit  the  foliate  pa- 
pillae are  especially  well  developed  and  are  abundantly 
supplied  with  taste-buds.  These  have  been  very  fully 
studied  recently  by  Heidenhain  (1914)  who  has  shown 
that  the  buds  are  arranged  in  more  or  less  vertical  rows 
on  each  papillar  fold  and  that  they  probably  increase 
in  numbers  by  a  process  of  fission. 

The  vallate  papillae,  which  in  man  are  usually  six  to 
twelve  in  number,  form  on  the  posterior  part  of  the 



tongue  a  V-shaped  row  whose  angle  points  toward  the 
esophagus  (See  Fig.  25).  Each  papilla  is  a  low  circu- 
lar elevation  surrounded  by  a  relatively  deep,  narrow 
ditch.  The  taste-buds  are  located  on  the  walls  of  this 
ditch  and  chiefly  on  that  wall  which  forms  part  of  the 
papilla.  In  a  vertical  section  through  a  vallate  papilla, 
it  is  usual  to  see  on  the  side  of  the  ditch  formed  by  the 

FIG.  27. — Vertical  section  of  a  vallate  papilla  showing  taste-buds. 

papilla  from  ten  to  a  dozen  taste-buds  and  on  the  side 
away  from  that  structure  four  to  six  such  bodies 
(Pig.  27).  However,  as  Schwalbe  (1868)  long  ago 
pointed  out,  much  individual  variation  occurs  and  it  is, 
therefore,  very  difficult  even  to  estimate  with  any  degree 
of  accuracy  the  total  number  of  taste-buds  on  a  single 
papilla.  W.  Krause  (1876)  believed  the  number  for  a 
single  papilla  in  man  to  be  as  high  as  2500,  but  von  Wyss 
(1870)  placed  it  much  lower,  namely,  at  about  400.  Even 
these  figures  seemed  too  high  to  Graberg  (1899)  who  gave 
the  maximum  at  100  to  150  and  the  minimum  at  40  to  50. 
Heiderich  (1906)  made  a  close  count  on  92  papillae  .from 
human  beings  ranging  in  age  from  the  first  to  the  twen- 
tieth year  and  found  the  extreme  numbers  of  buds  to  a 


papilla   to   be    508    and   33   with   an   average   not   far 
from  250. 

2.  Comparative  Distribution  -of  Taste-buds.     Taste- 
buds,  like  the  olfactory  receptors,  require  a  moist  sur- 
face.    It  is,  therefore,  not  surprising  to  find  that  in  all 
air-inhabiting  vertebrates  they  are  limited  to  the  oral 
cavity.     Their  distribution  in  mammals  has  been  very 
fully  studied  by  Tuckerman  (1892),  Munch  (1896),  and 
Haller  (1909). 

Taste-buds  also  appear  to  be  limited  to  the  oral 
region  in  amphibians  notwithstanding  the  fact  that  many 
of  these  animals  possess  a  permanently  moist  skin.  In 
fishes  they  were  apparently  first  seen  by  Leydig  in  1851 
and  were  subsequently  described  by  Schulze  (1863).  In 
these  forms  they  are  not  restricted  to  the  oral  region. 
According  to  Johnston  (1906)  they  are  present  on  the 
heads  of  cyclostomes  as  well  as  on  those  of  ganoids  where 
they  were  studied  by  Dogiel  (1897).  Herrick  (1918) 
states  that  in  some  bony  fishes,  such  as  the  catfishes,  the 
carps,  and  the  suckers  they  are  to  be  found  over  the 
entire  outer  surface  of  the  body  and  this  investigator 
(1903)  has  further  shown  that  in  the  catfish  Amiurus  the 
taste-buds  on  the  flank  of  the  fish  are  as  significant  in  the 
detection  of  bait  as  are  those  about  the  mouth  (Fig.  28). 

3.  General  Form  of  Taste-buds.    Taste-buds  vary  in 
form  from  that  of  a  flask  to  that  of  a.  spindle.     Commonly 
they  are  single  bud-shaped  bodies  opening  to  the  exte- 
rior by  a  small  pore  (Fig.  29).     Compound  buds  in  which 
the  body  of  the  bud  appears  double  and  two  pores  are 
present  have  long  been  known  and  Heidenhain  (1914)  has 
recently  shown  that  this  condition  may  reach  an  extreme 
degree  of  complexity  in  the  foliate  papillae  of  the  rabbit 


where  compound  buds  with  as  many  as  six  pores  have 
been  identified.  The  frequency  with  which  types  of  buds 
with  different  numbers  of  pores  occur  may  be  gathered 
from  the  enumeration  by  Heidenhain  who  found  that  in 
509  taste-buds  from  the  foliate  papillfe  of  the  rabbit 
368  had  one  pore,  100  two  pores,  29  three  pores,  7  four 

Fio.    28. — Lateral  view  of    the  catfish,  Amiurus   melas,  showing  in  black  the  gustatory 
branches  of  the  facial  nerve.     After  Herrick,  1903,  Fig.  3. 

pores,  1  five  pores,  and  4  six  pores.  In  the  compound 
buds  the  pores  usually  form  a  more  or  less  linear  series 
and  as  each  pore  represents  a  single  element  in  the  com- 
plex the  whole  gives  the  impression  of  a  row  of  fused 
buds  (Fig.  30.)  These  compound  buds  are  believed  to 
result  from  a  process  of  imperfect  division. 

Some  taste-buds  open  directly  on  the  oral  surface 
where  they  are  located ;  others  are  marked  by  a  pore,  the 
outer  taste-pore,  which  leads  into  a  short  canal  and  this 
in  turn  ends  at  the  inner  taste-pore  formed  by  the  distal 
end  of  the  bud  itself.  Von  Ebner  (1897)  noted  that  in 
some  instances  the  canal  expanded  into  a  small  chamber 
or  ampulla  over  the  tip  of  the  bud  and,  though  Grabcrg 
(1899)  could  not.  confirm  this  statement  for  man,  the 
condition  has  been  observed  anew  by  Kallius  (1905)  in 


human  material  and  by  Heidenhain  (1914)  in  the  rabbit. 

5.  Cellular  Composition  of  Taste-buds.  The  cells 
composing  the  taste-buds  are  so  arranged  as  to  give  each 
bud  somewhat  the  appearance  of  a  flower  bud  or  of  a 
leaf  bud  not  yet  unfolded.  As  has  been  stated  already, 
these  end-organs  were  described  in  the  skin  of  fishes  as 
early  as  1851  by  Leydig  and  were 
subsequently  simultaneously  and 
independently  discovered  in  the 
mouths  of  the  higher  vertebrates 
<by  Loven  (1867)  and  by  Schwalbe 
(1867).  The  older  workers 
usually  distinguished  in  the  taste- 
buds  two  classes  of  cells,  taste- 
cells,  which  were  supposed  to  be 
chiefly  central  in  position,  and 
supporting  cells  mainly  on  the 
exterior  of  the  bud. 

Each  taste-cell  is  an  attenuated  delicate  structure 
whose  elongate  nucleus  forms  a  slight  enlargement  near 
the  middle  of  the  cell-body  (See  Fig.  31a).  Distal  to 
it  narrows  to  a  delicate  process,  the  taste  hair.  This 
hair  either  projects  out  of  the  pore  into  the  exterior  or 
into  the  canal  when  that  is  present.  Proximal  to  the 
nucleus  the  taste-cell  extends  into  the  deeper  part  of  the 
bud  there  to  terminate  usually  in  a  small  rounded  knob. 
The  number  of  taste-cells  in  a  bud  varies  from  two  or 
three  to  as  many  as  the  contained  supporting  cells,  per- 
haps ten  or  more. 

Beside  the  taste-cells  proper  Schwalbe  (1867)  de- 
scribed what  he  believed  to  be  a  second  form  of  receptive 

Fio.  29. — A  simple  taste-bud 
from  a  foliate  papilla  of  the 
rabbit.  After  Heidenhain,  1914, 
Plate  19,  Fig.  5. 



Fio.  30. — A  compound]  taste- 
bud  from  a  foliate  papilla  of  the 
rabbit.  After  Heidenhain  1914,  Plate 
23,  Fig.  27. 

cell  to  which  he  gave  the  name  of  "Stabzelle"  or  rod  cell. 
This  type  of  cell  was  said  to  differ  from  the  ordinary 
taste  cell  in  that  it  was  without  a  taste  hair.  It  has  not  been 
identified  with  certainty  by  subsequent  investigators. 

The  supporting  cells  of  the  taste-buds  have  been  the 
occasion    of    much    difference    of    opinion.     The    older 

workers  believed  that  these 
cells  were  limited  to  the  exte- 
rior of  the  buds,  but  Merkel 
(1880)  showed  that  they  also 
occurred  in  the  interior  and 
Eanvier  (1888)  .  definitely 
described  both  inner  and  outer 
supporting  cells.  Hermann 
(1889)  concluded  that  these 
two  classes  of  supporting  cells 
differed  not  only  in  position  but  also  in  structure. 
The  outer  cells,  which  he  called  pier  cells  (Pfeilerzellen), 
were  relatively  large  pyramidal  elements  whose 
nuclei  were  proximal  in  location  and  whose  distal 
ends  terminated  in  a  zone  marked  with  fine  vertical 
stripings.  For  the  inner  supporting  cells  Hermann  used 
Schwalbe's  term  of  rod  cells  (Stabzellen)  without,  how- 
ever, wishing  thereby  to  imply  that  they  were  of  a  sensory 
nature.  They  were  said  to  differ  from  the  pier  cells 
in  that  they  were  devoid  of  the  peripheral  striped  zone. 
Hermann  also  described  basal  supporting  cells  which  to 
the  number  of  two  to  four  were  found  in  the  proximal 
part  of  the  taste-buds.  Von  Lenhossek  (1893b)  doubted 
the  existence  of  basal  cells  and  described  four  not  very 
sharply  separate  types  of  supporting  cells.  Graberg 
(1899)  reidentified  in  human  material  the  basal  cells  dis- 


covered  by  Hermann.  The  other  supporting  cells  were 
described  by  this  author  as  either  central  or  peripheral 
and  were  to  be  distinguished  from  each  other  rather  by 
location  than  by  differences  of  structure. 

The  indefiniteness  and  uncertainty  that  surrounded 
the  question  of  the  classes  of  supporting  cells  in  taste- 
buds  has  been  dissipated  in  large  part  by  the  declaration 
of  Kolmer  (1910)  that  between  the  taste-cells  on  one 
hand  and  the  so-called  supporting  cells  on  the  other  there 
are  all  possible  transitions  and  that  it  is,  therefore,  a 
mistake  to  attempt  to  draw  distinctions  not  only  between 
various  kinds  of  supporting  cells  but  between  supporting 
cells  and  taste-cells.  Kolmer  believed  that  all  the  elon- 
gated cells  in  taste-buds  are  really  taste-cells  and  that 
their  differences  are  due  to  the  stage  of  growth  at  which 
they  are  for  the  moment.  This  opinion,  which  is  sup- 
ported by  what  is  known  of  the  innervation  of  the  taste- 
buds,  has  gained  the  acceptance  of  the  more  important 
recent  workers  in  this  field,  such  as  Retzius  (1912)  and 
Heidenhain  (1914).  If  true,  it  shows  the  taste-bud  to  be 
a  much  more  unified  structure  than  has  heretofore  been 
supposed  and  it  does  away  at  once  with  the  confusion 
over  the  classes  of  cells  that  were  believed  to  enter  into 
its  composition. 

The  basal  cells  apparently  do  not  fall  into  this  general 
category  of  more  or  less  differentiated  receptor  cells, 
but,  according  to  Heidenhain  at  least,  they  are  elements 
that  only  under  certain  conditions  are  regularly  present 
and  are  concerned  with  the  division  of  the  buds. 

The  epidermal  cells  immediately  next  the  taste-bud 
are  often  flattened  against  this  structure  and  conform 
more  or  less  to  its  outline.  These  cells  have  been  called 



by  Grabcrg  (1899)  extrabulbar  cells  and  though  they  are 
not  to  be  classed  as  part  of  the  bulb  proper  they  are 
nevertheless  sufficiently  related  to  that  structure  to  be 
appropriately  mentioned  in  this  connection. 

As  Hermann  (1889)  long  ago  pointed  out,  the  cells  of 
the  taste-buds  are  probably  undergoing  continual  change. 
Old  cells  are  degenerating  and  disappearing  and  new 
ones  are  forming  to  take  the  places  of  those  that  have 
broken  down.  The  degenerating  process  is  indicated  by 
the  presence  in  the  taste-buds  of  cells  in  all  stages  of 
growth  and  of  considerable  numbers  of  leucocytes,  as 
pointed  out  by  Ranvier  (1888),  von  Lenhossek  (1893b), 
and  others.  The  regenerative  process  is  shown  in  the 
occasional  occurrence  of  mitotic  figures  in  the  base  of 
the  bud  thus  giving  evidence  of  cell  division  in  that  region 
Hermann  (1889). 

5.  Intragemmal  and  other  Spaces.     Graberg  (1899) 
has  called  attention  to  the  fact  that  taste-buds  are  not 
solid  structures  but  that  their  cells  are  separated  one  from 
another   by   considerable   intervening   space,    and   that 
much  free  space  occurs  in  the  tissue  immediately  around 
the  buds.    This  intra-,  peri-,  and  subgemmal  space  is  be- 
lieved by  Graberg  not  to  be  an  artifact,  for  it  can  be 
identified  by  almost  all  methods  of  preparation.  Accord- 
ing to  this  investigator  these  various  spaces  communicate 
with  one  another  and  connect  with  the  exterior  through 
the  taste  pore.     They  may  be  the  means  of  irrigating  and 
thereby  cleaning  the  taste-bud,  for  it  is  possible  that 
fluid  may  flow  slowly  through  them  from  the  interior  to 
the  exterior. 

6.  Innervation  of  Taste-buds.     Among  the  older  in- 
vestigators the  innervation  of  the  taste-buds  was  a  ques- 


tion  of  much  uncertainty.  Some  claimed  that  the 
gustatory  nerve-fibers  connected  directly  with  the  cells 
of  the  taste-buds;  others  that  they  did  not  so  connect. 
The  first  to  employ  special  neurological  methods  for  the 
solution  of  this  question  were  Fusari  and  Panasci  (1890). 
These  workers  claimed  that  by  means  of  Golgi  prep- 

FIG.  31. — Golgi  preparations  of  the  taste-buds  of  the  rabbit,  a  showing  cells  (after 
von  Lenhosse'k,  1893a,  Fig.  la)  and  6  showing  nerve-terminations  (after  Retzius,  1892a, 
PlateS,  Fig.  4). 

arations  it  could  be  shown  that  the  gustatory  cells  were 
directly  connected  with  nerve-fibers.  Two  years  later 
Retzius  (1892a)  published  an  account  of  the  innervation  of 
the  taste-buds  of  mammals  and  of  amphibians  in  which 
he  showed  in  preparations  stained  by  methylenblue  as 
well  as  by  the  Golgi  process  that  the  nerve-fibers  were 
not  directly  connected  with  the  taste-cells  but  ended  in 
close  proximity  to  them  (Fig.  31).  These  results  were 
confirmed  in  189  3  by  von  Lenhossek,  Arnstein,  and  Jacques 
as  well  as  by  the  subsequent  work  of  Retzius  himself 
(1893)  and  there  seems  to  be  no  ground  for  doubting 
the  correctness  of  the  general  conclusion  arrived  at  more 
or  less  independently  by  these  four  investigators. 

The  anatomical  relations  shown  by  these  workers 
are  relatively  simple.  From  the  subepithelial  nerve 
plexus  in  the  neighborhood  of  taste-buds  fibers  pass  out- 
ward into  the  epidermis.  These  fibers  either  form  sys- 



terns  of  branches  ending  in  free  terminations  around  a 
taste-bud,  in  which  case  they  are  called  perigemmal  or 

Fio.  32. — Golgi  preparations  of  the  taste-buds  of  the  cat,  a,  in  longitudinal  section 
fihowing  nerve  terminations,  and  6,  in  transverse  section  showing  intrageinmal  nerve 
ebers.  After  Retzius,  1892a,  Plate  7,  Figs.  1  and  4. 

peribulbar  fibers,  or  they  enter  the  bud  and  end  freely 
among  its  cells  being  designated  then  as  intragemmal 
or  intrabulbar  fibers  (Fig.  32).  As  the  figures  given  by 
Retzius,  Arnstein,  and  others  show,  the  nerve-fibers  in 


the  buds  are  as  intimately  applied  to  the  so-called  sup- 
porting cells  as  to  the  taste-cells,  showing,  as  has  al- 
ready been  stated,  that  the  distinction  between  what 
has  been  assumed  to  be  two  classes  of  cells  is  probably 
quite  erroneous. 

In  addition  to  intergemmal  and  iperigemmal  fibers, 
which  in  consequence  of  their  close  relations  with  the 
taste-buds  may  be  designated  as  gemmal  or  bulbar  fibers, 
there  are  also  fibers  that  pass  into  the  undifferentiated 
epithelium  between  the  buds  and  end  close  to  the  external 
surface  as  free-nerve  terminations.  These  have  been 
called  intergemmal  fibers,  but  it  is  doubtful  whether 
they  have  anything  to  do  with  taste  and  it  is  not  improb- 
able that  they  are  concerned  with  other  sensory  functions 
such  as  the  common  chemical  sense,  pain,  and  the  like,  in 
which  case  a  designation  implying  relations  to  a  taste-bud 
is  in  no  sense  appropriate. 

Taste-buds  such  as  have  already  been  described  have 
been  found  in  a  wide  range  of  vertebrates.  They 
not  only  occur  in  mammals,  where  their  relation  with 
the  nerve-fibers  was  first  correctly  described  by  Retzius 
( 1892a) ,  but  also  in  fishes  as  seen  by  Retzius  ( 1892a,  1893 ) , 
vonLenhossek  (1893a),Dogiel  (1897) and  others  (Fig.  33). 
It  is,  therefore,  probable  that  so  far  as  essentials  are  con- 
cerned the  innervation  of  the  taste-buds  of  all  vertebrates 
presents  a  relatively  uniform  plan. 

7.  Gustatory  Nerves.  There  are  no  separate  gusta- 
tory nerves  in  the  vertebrates  as  there  are  olfactory  nerves 
or  optic  nerves.  Gustatory  fibers  occur  in  several  crani- 
al nerves  and  it  is  by  means  of  these  that  the  taste-buds 
of  various  regions  are  provided  with  those  nervous  con- 
nections that  have  been  described  in  the  preceding  section. 


In  the  fishes  the  nerves  chiefly  concerned  are  the  vagus, 
the  glossopharyngeal  and  the  facial.  The  taste-buds  of 
the  gill  region  are  supplied  by  the  vagus  and  the  glosso- 
pharyngeal. Those  that  are  in  the  mouth  proper  or  are 
on  the  exterior  of  the  body  are  innervated  by  the  facial 
ni»  1-ve.  Consequently  in  the  catfish  (See  Fig.  28),  in 

which  the  whole  outer 
skin  is  provided  with 
taste-buds,  this  nerve  is 
enormously  developed  and 
sends  large  branches  to 
the  barbels  and  an  exten- 
sive recurrent  branch  to 
the  flanks  of  the  body 
(Herrick,  1903). 

In    mammals,    includ- 


Hlg    mail,     tllC    llinerVatlOll 

of  the  taste-buds  is  not 
upon  so  simple  a  plan  as  in  fishes.  In  these  higher 
vertebrates  gustatory  fibers  may  possibly  be  contained 
in  four  of  the  cranial  nerves,  the  vagus,  the  glosso- 
pharyngeal, the  facial,  and  the  trigeminal.  The 
distribution  of  these  nerves  in  the  human  tongue  has  been 
worked  out  by  Zander  (1897).  Certain  parts  of  the 
vagus  are  distributed  to  the  larynx  and  to  the  epiglottis 
as  well  as  to  the  most  posterior  part  of  the  tongue  itself 
and  innervate  very  probably  the  taste-buds  of  these  re- 
gions (Fig.  34).  The  glossophaiyngeal  supplies  the  pos- 
terior third  of  the  tongue  including  the  foliate  and  vallate 
papillae,  for,  as  was  first  shown  by  von  Vintschgau  and 
Honigsf.hmied  (1876),  when  the  ninth  nerve  is  cut  the 
taste-buds  of  these  parts  soon  degenerate  and  disappear. 
Although  the  correctness  of  this  observation  was  denied 

Fio.  33.  —  Golgi  preparations  of  the  taste- 
buds  of  the  common  European  barbel  show- 
ing  cells  and  nerve-fibers.  After  von  Lenhossek, 

1893  a.  Fig.  2. 



by  Baginsky  (1894),  it  has  been  confirmed  by  such  a 
number  of  observers,  including  Drasch  (1887),  Ranvier 
(1888),  Sandmeyer  (1895),  Meyer  (1897)  and  others,  that 
it  is  now  generally  accepted.  Both  the  right  and  the 
left  branches  of  this  nerve  innervate  the  median  vallate 
papilla  in  mammals  and  form  at  the  base  of  this  organ, 
as  Vastarini-Cresi  (1915)  has 
shown,  more  or  less  of  a  gusta- 
tory chiasma.  The  anterior  two- 
thirds  of  the  tongue  in  man  are 
innervated  by  the  lingual  nerve 
which  is  made  up  of  a  union  of 
the  lingual  branch  of  the  trigem- 
inal  nerve  with  the  chorda 
tympani  of  the  facial.  It  has 
been  an  open  question  whether 
the  gustatory  fibers  for  this  part 
of  the  tongue  belong  to  the 
trigeminal,  to  the  facial,  or 
possibly  even  to  the  glosso- 
pharyngeal,  for  all  these  nerves 
intercommunicate  through  a 
plexus  of  fine  branches  near  their  roots.  F.  Krause  (1895) 
noted  the  effect  on  taste  of  the  complete  extirpation  of  the 
ganglion  of  the  trigeminal  nerve,  and  found  that  in  some 
instances  taste  was  entirely  obliterated  from  the  appro- 
priate part  of  the  tongue,  but  that  in  others  it  was  only 
somewhat  reduced.  These  differences  do  not  appear  in 
the  later  and  more  conclusive  work  of  Gushing  (1903) 
who  found  that,  when  time  enough  was  given,  all  subjects 
from  whom  the  ganglion  of  the  trigeminal  nerve  had 
been  removed,  recovered  taste  completely.  He  attributed 

FIG.  34. — Diagram  of  the 
human  tongue  showing  the  parts 
innervated  by  the  lingual  nerve 
(horizontal  lines),  by  the  glosso 
pharyngeal  nerve  (oblique  lines), 
and  by  the  vagus  nerve  (small 
circles).  After  Zander,  1897. 



the  temporary  disturbance  in  taste,  a  condition  that  was 
supposed  to  be  permanent  by  Krause,  to  the  effect  of  the 
degenerating  trigeminal  fibers  on  the  adjacent  gusta- 
tory fibers,  an  effect  that  disappeared  when  the  degenera- 
tion was  complete.  Consequently  Gushing  concluded  that 
the  gustatory  fibers  from  the  anterior  part  of  the  tongue 


FIG.  35. — Diagram  to  illustrate  the  possible  paths  of  the  gustatory  nerve-fibers  from  the 
tongue  to  the  brain  in  man.  The  distal  part  of  the  tongue  (1)  is  innervated  by  the  lingual 
nerve  (2)  whose  gustatory  fibers  pass  to  the  brain  by  way  of  the  chorda  tympani  (3),  a 
branch  of  the  facial  nerve  (VII).  The  proximal  part  of  the  tongue  is  innervated^  by  the 
glossopharyngeal  nerve  (4).  The  undoubted  gustatory  paths  over  the  facial  nerve  (VII)  and 
the  glossopharyngeal  nerve  (IX)  are  indicated  by  dotted  lines.  The  commonly  assumed 

Eaths  by  way  of  the  trigeminal  nerve  (V)  are  shown  in  heavy  black  lines  with  arrows.    Modi- 
ed  from  Cushing,  1903. 

are  not  part  of  the  trigeminal  nerve.  If  this  is  so,  they 
must  belong  to  the  facial  or  possibly  to  the  glossopharyn- 
geal nerve  (Fig.  35).  That  they  are  abundantly  present 
in  the  chorda  tympani  of  the  facial  nerve  is  well  known 
from  the  fact  that  direct  stimulation  of  the  chorda  in 
the  neighborhood  of  the  ear  drum  is  commonly  accom- 
panied by  sensations  of  taste,  but  whether  these  gusta- 
tory fibers  on  reaching  the  facial  nerve  pass  into  the  brain 
through  its  root  or  make  their  way  to  the  root  of  the  glos- 
sopharyngeal is  not  yet  definitely  settled.  It  is,  therefore, 
probable  that  in  mammals  the  trigeminal  nerve,  though 


suspected  of  including  gustatory  fibers,  is  really  devoid 
of  them.  These  fibers  at  most  occur  in  the  facial,  glos- 
sopharyngeal  and  vagus  nerves,  but  none  of  these  nerves 
is  exclusively  gustatory. 

8.  Eelation  of  Gustatory  Nerve-fibers  to  Taste-buds. 
It  is  an  interesting  and  significant  fact  that  on  the  de- 
generation of  the  gustatory  nerve-fibers  the  taste-buds 
associated  with  them  should  disappear.  This  state  of 
affairs,  long  ago  demonstrated  for  mammals,  has  recently 
been  shown  by  Olmsted  (1920a,  1920b)  to  occur  also  in 
fishes.  Meyer  (1897)  showed  that  thirty  hours  after  cut- 
ting the  glossopharyngeal  nerve  in  the  rabbit  the  taste- 
buds  began  to  show  a  change  and  that  by  the  end  of  seven 
days  most  of  them  had  disappeared.  In  the  catfish  Ami- 
urus, according  to  Olmsted,  the  taste-buds  on  the  oral 
barbels  begin  to  degenerate  in  a  little  over  ten  days  after 
the  nerve  to  these  organs  has  been  cut  and  they  com- 
pletely disappear  by  the  end  of  the  thirteenth  day. 
Ranvier  (1888)  believed  that  in  mammals  the  taste-buds 
were  destroyed  by  wandering  cells,  but  Sandmeyer  (1895) 
and  Meyer  (1897)  held  the  view  that  the  gustatory  cells 
suffered  dedifferentiation  and  changed  into  ordinary  epi- 
thelial cells.  In  Amiurus  Olmsted  has  found  strong 
evidence  in  favor  of  the  destruction  of  the  cells  of  the 
taste-buds  by  phagocytes  thus  supporting  Ranvier 's 
original  opinion. 

Olmsted  has  shown,  further,  that  on  the  regeneration 
of  a  nerve  in  a  denervated  Amiurus  barbel  from  which 
all  the  taste-buds  had  disappeared,  new  buds  reappear 
coincident  with  the  arrival  of  the  nerve.  With  the  de- 
generation of  the  nerve  and  the  loss  of  the  taste-buds  the 
barbels  lose  their  receptivity  for  sapid  materials,  nor 


does  this  return  till  the  buds  regenerate.  Since  the  taste- 
buds  disintegrate  with  the  loss  of  the  nerve  and  new  ones 
form  only  with  the  regeneration  of  this  structure,  it  is 
clear  that  the  bud  is  dependent  upon  the  nerve.  As 
Olmsted  has  suggested,  it  is  probable  that  when  a  twig  of 
the  nerve  reaches  a  given  spot  in  the  epidermis,  it  gives 
out  a  substance,  hormone-like  in  character,  that  excites 
the  epithelial  cells  of  that  spot  to  form  a  bud  much  as  the 
embryonic  eye  cup  of  the  vertebrate  excites  in  the  super- 
imposed ectoderm  the  formation  of  a  lens.  In  this  way  at 
least  the  intimate  dependence  of  the  taste-bud  on  the  re- 
generating nerve-fiber  can  be  explained  and,,  judging 
from  the  account  given  by  Landacre  (1907)  of  the  ontoge- 
ny of  these  organs,  a  similar  explanation  may  also 
apply  in  development. 


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Contents. — 1.  Location  of  Taste.  2.  Gustatory  stimulus. 
3.  Qualities  of  Taste.  4.  The  Sour  Taste.  5.  The  Saline 
Taste.  6.  The  Bitter  Taste.  7.  The  Sweet  Taste.  8. 
Inadequate  Stimuli.  9.  Distribution  of  Tastes  on  the 
Tongue.  10.  Action  of  Drugs  on  Taste.  11.  Substances 
with  two  Tastes.  12.  Latency  of  Taste  Sensations.  13. 
Taste  Alterations ;  After-tastes.  14.  Gustatory  Contrasts. 
15.  Taste  Compensations  and  Mixtures.  16.  The  Gusta- 
tory Senses.  17.  Comparative.  18.  Bibliography. 

1.  LOCATION  of  Taste.  Although  in  man  taste  is  not 
strictly  limited  to  the  mouth,  for  it  spreads  into  some 
of  the  adjacent  cavities,  it  is  primarily  located  in  the 
buccal  space  and  is  especially  a  function  of  the  tongue. 
When  the  mouth  of  a  normal  adult  is  explored  witli  solu- 
tions of  sapid  substances,  many  parts  such  as  the  lips, 
the  gums,  the  floor,  the  lower  surface  of  the  tongue,  the 
inner  surfaces  of  the  cheeks,  and  the  hard  palate  are 
found  to  be  insensitive  to  taste.  Even  the  uvula  which, 
according  to  many  of  the  older  workers,  was  regarded  as 
having  a  gustatory  function,  has  been  shown  by  Kiesow 
and  Hahn  (1901)  not  to  be  concerned  with  taste.  All 
these  regions  are  well  known  to  be  devoid  of  taste-buds. 
Whether  the  pillars  of  the  fauces  and  the  tonsils  have 
to  do  with  taste  is  a  matter  of  dispute.  Hanig  (1901) 
believed  that  these  parts  have  a  gustatory  function,  but 
Kiesow  and  Hahn  (1901)  regarded  them  as  usually  in- 
sensitive. The  mucous  membranes  of  the  following  parts 



are  concerned  with  taste;  the  beginning  of  the  gullet, 
the  region  of  the  arytenoid  cartilages  within  the  larynx, 
the  epiglottis,  the  soft  palate,  and  particularly  the  tongue. 
In  all  these  regions  taste-buds  have  been  identified.  On 
the  tongue  of  adult  human  beings  taste  is  limited  to  the 
tip,  the  lateral  margins  and  the  dorsal  surface  of  the 
root,  the  large  central  area  on  the  upper  surface  of  this 
organ  being  devoid  of  taste.  In  children,  as  contrasted 
with  adults,  the  whole  upper  surface  of  the  tongue  in- 
cluding the  central  area  is  said  to  be  sensitive  to  taste 
as  is  also  the  inner  surfaces  of  the  cheeks. 

2.  Gustatory  stimuli.  The  stimulus  for  taste  is  an 
aqueous  solution  of  a  great  variety  of  substances.  Mate- 
rials insoluble  in  water  are  tasteless,,  but  not  all  substances 
that  form  aqueous  solutions  have  taste.  Thus  oxygen, 
hydrogen,  and  nitrogen,  though  freely  soluble  in  water, 
are  without  taste.  Piutti  (1886)  long  ago  showed  that 
Isevo-asparagine  is  tasteless,  although  its  stereoisomer 
dextro-asparagine  is  sweet.  Other  organic  compounds, 
such  as  the  carbohydrates  raffinose  and  alpha-galaoctite 
are  said  to  be  almost,  if  not  quite,  tasteless. 

When  solids  or  semi-solids  are  chewed  in  the  mouth, 
they  not  only  become  mixed  with  the  saliva  whereby  many 
of  their  components  become  dissolved,  but  they  are 
spread  over  the  surface  of  the  tongue  and  are  thus 
brought  into  intimate  contact  with  its  taste-buds.  In  fact 
it  is  not  improbable  that  the  movement  of  the  tongue  fa- 
cilitates the  entrance  of  these  solutions  into  the  pores  of 
the  taste-buds.  At  least  solutions  placed  upon  the 
tongue,  particularly  near  its  root,  are  tasted  with  greater 
certainty,  when  this  organ  is  moved  about  than  when  it 
is  held  still. 


3.  Qualities  of  Taste.  Tastes,  unlike  odors,  fall  into 
a  limited  number  of  well-circumscribed  groups,  which 
have  received  distinctive  names  such  as  sour,  saline,  bit- 
ter, sweet,  and  the  like.  The  multitude  of  flavors  and 
other  sensations  associated  with  our  food  are  undoubt- 
edly mixed  in  character  and  include  touch,  heat,  cold,  the 
common  chemical  sensation,  and  especially  odor.  By  ap- 
plying materials  in  weak  solution,  at  the  temperature  of 
the  mouth  and  with  the  nostrils  closed,  extraneous  sen- 
sations may  be  eliminated  and  there  remains  a  certain 
irreducible  residue,  the  tastes.  Zenneck  (1839),  Valentin 
(1848),  Duval  (1872)  and  later  Sternberg  (1898)  admit- 
ted only  two  classes  of  tastes,  sweet  and  bitter.  Stich 
(1857),  however,  long  ago  showed  that  sour  was  a  sensa- 
tion produced  by  stimulating  only  a  limited  part  of  the 
buccal  surface,  and  Schiff  (1867)  made  the  important 
observation  that  a  solution  of  acid  too  weak  to  stimulate 
the  general  mucous  surface  would  nevertheless  call  forth 
a  sour  sensation  when  it  was  applied  to  the  gustatory 
region.  Von  Vintschgau  (1880)  made  similar  observa- 
tions concerning  the  saline  taste;  solutions  of  sodium 
chloride,  potassium  iodide,  and  ammonium  chloride,  if 
sufficiently  weak,  will  stimulate  the  organs  of  taste,  but 
if  strong  they  will  stimulate  not  only  these  organs  but 
the  nerve  endings  of  the  general  buccal  cavity  also.  In 
consequence  of  such  observations  sour  and  saline  are  now 
universally  included  with  bitter  and  sweet  as  true  tastes. 

In  addition  to  these  four  tastes  there  are  a  number 
of  questionable  ones  such  as  metallic  and  alkaline,  tastes 
that  were  originally  accepted  by  Wundt  (1887)  among 
others.  The  so-called  metallic  taste  is  excited  by  solu- 
tions of  salts  of  the  heavy  metals,  silver,  mercury,  and 


the  like  (Kahlenberg,  1898).  The  metallic  taste  of  a 
0.0005  molar  solution  of  silver  nitrate  is  very  pronounced 
and  is  discernible  even  at  the  greater  dilution,  0.0002. 
Since  the  nitrate  ions  are  incapable  of  exciting  taste  at 
such  slight  concentrations,  it  follows  that  stimulation 
must  depend  upon  the  silver  ions.  In  a  similar  way  mer- 
cury ions  in  normal  solutions  of  0.001  to  0.0005  of  mercu- 
ric chloride  have  been  shown  to  excite  the  so-called 
metallic  taste.  This  taste,  however,  has  been  declared 
to  be  a  complex  of  other  tastes  such  as  sour  and  sweet, 
and  Herlitzka  (1808)  has  gone  so  far  as  to  maintain  that 
it  is  not  a  true  taste  but  an  olfactory  phenomenon. 

The  alkaline  tastes  so-called  are  excited  by  the  appli- 
cation to  the  tongue  of  dilute  solutions  of  such  caustic 
alkalis  as  sodium  or  potassium  hydrate.  Kahlenberg 
(1898)  has  shown  that  the  stimulating  material  in  such 
mixtures  is  the  hydroxylion  which  is  effective  in  solutions 
as  weak  as  0.0025  molar.  In  the  alkaline  taste,  as  in  the 
metallic  taste,  the  results  have  been  variously  explained. 
Oehrwall  (1891)  regarded  the  so-called  alkaline  taste  as 
a  mixture  of  sensations  due  to  a  simultaneous  combina- 
tion of  several  tastes  and  touch.  Hober  and  Kiesow 
(1898)  pointed  out  that  weak  alkalis  produce  a  sweetish 
taste,  but  von  Frey  (1910)  showed  that  these  reagents 
act  on  the  tongue  in  such  a  way  as  to  produce  odorous 
materials  that  he  believed  to  be  the  occasion  of  the  so- 
called  alkaline  taste.  He,  therefore,  relegated  these  as- 
sumed tastes  to  olfaction. 

Insipidity,  such  as  is  characteristic  of  distilled  water, 
is  probably  real  tastelessness.  Oehrwall  (1891)  attributed 
it  to  the  absence  of  small  amounts  of  carbon  dioxide  from 
such  waters  and  this  is  probably  true,  for  tastelessness 


disappears  on  the  addition  of  some  of  this  gas  to  insipid 
water.  Henle  (1880)  showed  that  insipidity  was  char- 
acteristic of  fluids  that  contained  less  salt  than  the  saliva. 
Insipidity  is  probably  a  deficiency  phenomenon  and  may 
be  produced  by  the  absence  of  several  classes  of  sub- 
stances. Nevertheless  it  must  not  be  forgotten  that  a 
condition  of  staleness  or  flatness  in  water,  practically 
indistinguishable  from  insipidity,  can  be  produced  by 
introducing  into  the  water  very  small  amounts  of  caustic 
alkali  whereby  hydroxyl  ions  are  liberated  (Kiesow, 

4.  The  Sour  Taste.  Sour  taste  has  long  been  asso- 
ciated with  acid  substances.  In  fact  it  seems  very  prob- 
able that  the  sour  taste  is  excited  only  by  acids,  acid 
salts,  or  materials  that  produce  acids.  All  these  sub- 
stances on  going  into  aqueous  solution  give  rise  to  hydro- 
gen ions  by  the  dissociation  of  acid  molecules.  If  the 
solutions  are  strong  they  will  also  contain  a  certain 
number  of  undissociated  acid  molecules.  It  was  pointed 
out  by  Richards  (1898)  that,  since  all  such  solutions  have 
the  sour  taste  and  since  the  one  component  that  they  all 
have  in  common  is  the  hydrogen  ion,  this  ion  must  be  the 
occasion  of  their  common  taste.  This  conclusion  was 
independently  arrived  at  in  another  way  by  Kahlenberg 
(1898).  A  0.0025  molar  solution  of  hydrochloric  acid  has 
a  pronounced  sour  taste  and  its  dissociation  into  hydro- 
gen and  chlorine  ions  is  practically  complete.  A  corres- 
ponding solution  of  sodium  chloride  is  also  about 
completely  dissociated  into  sodium  and  chlorine  ions  but 
is  without  taste.  It  follows,  therefore,  since  there  are 
as  many  chlorine  ions  in  the  salt  solution  as  in  the  acid 
solution  per  unit  volume  and  the  salt  solution  is  without 


taste,  that  the  sour  taste  of  the  acid  solution  cannot  be 
due  to  its  chlorine  ions  but  must  be  occasioned  by  its  only 
other  constituent,  the  hydrogen  ions.  Kahlenberg,  there- 
fore, concluded  that  these  ions  are  accountable  for  the 
sour  taste. 

This  view  is  supported  by  the  fact  that  the  sourness 
of  all  acid  solutions  is  the  same,  for  instance,  it  is  impos- 
sible to  distinguish  by  taste  hydrochloric  acid  from  nitric 
or  sulphuric  acid.  So  far  as  the  sensations  are  concerned 
all  these  reagents  produce  identical  results,  the  one  qual- 
ity of  sourness.  There  has  been  some  tendency  to  sepa- 
rate astringency  from  sourness,  but  it  is  generally 
conceded  that  astringency  is  merely  sourness  near  the 
vanishing  point.  With  hydrochloric  and  other  mineral 
acids  this  occurs  in  molar  solutions  at  about  0.00125  to 
0.001  below  which  the  acid  solutions  cannot  be  distin- 
guished from  pure  water. 

From  this  standpoint  sour  taste  might  be  regarded 
as  due  directly  to  hydrogen  ions  and  the  intensity  of  this 
taste  to  depend  upon  the  concentration  of  such  ions. 
But  the  question  is  not  so  simple  as  this.  Although  solu- 
tions of  most  mineral  acids  agree  well  among  themselves 
so  far  as  sourness  and  hydrogen  ion  concentration  are 
concerned,  organic  acids  are  not  necessarily  so  related. 
Most  organic  acids  are  much  less  dissociated  in  aqueous 
solution  than  are  inorganic  acids  and  contain,  therefore, 
in  normal  solution,  fewer  hydrogen  ions  per  unit  volume, 
than  inorganic  acids  do.  Nevertheless  Eichards  (1898) 
found  that  tartaric,  citric,  and  especially  acetic  acids  were 
more  sour  than  would  have  been  expected  from  the  hydro- 
gen ion  concentration  of  their  solutions.  According  to 
Richards  acetic  acid  is  about  as  sour  as  a  solution  of 


hydrochloric  acid  one-third  as  concentrated.  Nevertheless 
the  acetic  acid  is  dissociated  only  about  one-fourteenth  as 
much  as  the  hydrochloric.  Hence  ion  for  ion  the  acetic 
acid  solution  is  the  more  sour  of  the  two.  This  result 
was  also  arrived  at  by  Kahlenbcrg  (1898)  who  estimated 
the  sourness  of  acetic  acid  at  a  concentration  of  0.005 
molar  to  be  about  four  times  what  should  be  expected  from 
its  hydrogen  ion  content.  These  differences  were  sub- 
sequently reaffirmed  by  Becker  and  Hertzog  (1907). 

It  is  by  no  means  easy  to  explain  the  excess  of  sour- 
ness on  the  part  of  acetic  and  other  like  acids.  Richards 
has  suggested,  without  putting  great  stress  on  the  idea, 
that  the  additional  sourness  of  acetic  acid  may  be  due  to 
the  undissociated  molecules,  which,  serving  as  a  reserve, 
producing  additional  hydrogen  ions  as  those  present  are 
used  up  in  the  reaction  between  the  acid  solution  and  the 
surface  of  the  receptor,  an  opinion  supported  by  the 
recent  work  of  Harvey  (1920).  Crozier  (1916,  1918a, 
1918b),  on  the  other  hand,  has  pointed  out  the  probability 
that  the  question  is  double,  one  part  having  to  do  with 
penetration  and  the  other  with  the  production  of  the 
sour  taste.  By  taking  advantage  of  natural  indica- 
tors, such  as  the  blue  pigment  in  the  integument  of 
Chrompdoris,  it  can  be  easily  shown  that  acids  pene- 
trate living  cells.  This  may  be  assumed  to  be  the 
first  step  in  sour  gustation.  But  penetration  observed 
in  this  way  is  a  much  slower  process  than  gustation, 
hence  the  penetration  concerned  with  taste  can  have 
to  do  only  with  the  most  superficial  layer  of  the 
taste  cells.  It  is  the  ease  of  combination  with  this  layer 
that  may  make  the  difference  between  acetic  acid  and 
other  acids.  Different  acids  having  penetrated  the  sur- 


faces  of  gustatory  cells  at  different  rates,  their  uniform 
sour  taste  may  then  be  ascribed  to  their  common  dissocia- 
tion product,  the  hydrogen  ion.  How  this  is  accomplished 
is,  according  to  Crozier,  the  second  problem  in  gustation. 
That  the  sour  taste  is  in  some  way  dependent  upon  hydro- 
gen ions  seems  true  beyond  reasonable  doubt.  How  these 
ions  become  effective  is  still  a  problem. 

5.  The  Saline  Taste.  The  saline  taste  is  typified  by 
that  of  common  salt.  Sodium  chloride,  however,  is  not 
the  only  substance  that  possesses  this  taste,  for  there  is 
a  whole  range  of  compounds  that  have  the  same  property. 
The  chlorides  of  potassium,  lithium,  ammonium,  and  mag- 
nesium, the  hydro  chlorides  of  monomethylamine  and  of 
diethylamine,  the  bromides  and  iodides  of  sodium  and  of 
potassium  as  well  as  their  sulphates  and  nitrates  are 
all  more  or  less  saline  in  taste. 

Aqueous  solutions  of  most  of  these  salts  show  a  high 
degree  of  dissociation  so  that,  beside  undissociated  mole- 
cules, cations  and  anions  are  present  in  these  solutions 
as  possible  stimuli  for  the  saline  taste.  Hober  and  Kiesow 
(1898)  have  worked  on  this  question  and  have  declared 
in  favor  of  ions  as  the  stimulating  agents  in  contrast  with 
undissociated  molecules.  Kahlenberg  (1898)  arrived  at 
the  same  conclusion.  He  found  that  a  solution  of  sodium 
chloride,  0.02  molar,  was  scarcely  to  be  distinguished  by 
taste  from  pure  water.  At  0.04  molar  it  was  a  trifle 
saline.  Corresponding  solutions  of  sodium  acetate  were 
almost  tasteless  and  certainly  not  in  the  least  saline. 
Hence  it  is  evident  that  the  salty  taste  of  sodium  chloride 
is  due  to  chlorine  ions  and  not  to  sodium  ions.  This  con- 
clusion is  supported  by  the  fact  that  0.04  molar  solutions 
of  potassium  chloride  and  of  lithium  chloride  are  also 


salty.  Other  chlorides,  such  as  those  of  ammonium  and 
magnesium,  have  a  saline  taste. 

This  taste,  however,  is  not  due  exclusively  to  chlorine 
ions.  Sodium  bromide  at  0.02  molar  has  a  faint  saline 
taste  and  is  unquestionably  salty  at  0.04.  Hence  the 
bromine  ion  must  also  be  a  stimulus  for  the  salty  taste. 
Kahlenberg  (1898)  reported  it  as  not  quite  so  effective 
in  this  respect  as  the  chlorine  ion.  Although  solutions 
of  sodium  iodide  at  0.04  or  even  at  0.02  molar  could  be 
distinguished  from  water,  they  did  not  give  an  unques- 
tionable taste  till  a  concentration  of  0.16  was  reached. 
At  this  concentration  the  taste  wras  markedly  saline.  A 
corresponding  solution  of  potassium  iodide  was  found 
also  to  be  salty  though  in  this  instance  the  taste  was  ac- 
companied by  a  slightly  bitter  flavor.  Prom  these  con- 
siderations it  is  evident  that  iodine  ions  are  saline  stimuli 
though  they  are  not  so  effective  in  this  respect  as  chlorine 
or  bromine  ions  are.  The  sulphates  of  sodium  and  of 
potassium  as  wrell  as  their  nitrates  also  have  a  saltiness 
in  their  tastes  and  it  has  been  shown  in  these  instances 
that  the  sulphate  and  nitrate  ions  are  the  effective  agents. 
Thus  all  saline  tastes  depend  upon  ionic  stimuli,  and,  as 
Kahlenberg  (1898)  and  Hober  and  Kiesow  (1898)  have 
maintained,  these  ions  are  always  anions,  a  conclusion 
supported  by  the  more  recent  work  of  Herlitzka  (1908). 

6.  The  Bitter  Taste.  The  bitter  taste  is  character- 
istic of  almost  all  alkaloids,  and  of  certain  unrelated  sub- 
stances such  as  dextro-mannose,  the  glucosides,  picric 
acid,  ether,  and  certain  inorganic  salts  such  as  magnesium 
sulphate  or  Epsom  salt. 

Magnesium  salts  when  sufficiently  concentrated  have 
a  bitter  taste  and  this  taste  is  due  to  the  magnesium  ion. 
This  is  in  strong  contrast  with  the  ions  of  sodium  and 


of  lithium,  which  are  apparently  almost  tasteless.  Am- 
monium and  calcium  ions  are  also  bitter  in  taste.  In 
picric  acid  the  sour  taste  of  the  hydrogen  ion  is  probably 
completely  masked  by  the  bitter  taste  of  the  picric  anion 
though  the  taste  of  this  substance  as  well  as  that  of  ether, 
dextro-mannose,  the  glucosides  and  other  such  substances 
appears  never  to  have  been  fully  investigated. 

But  the  substances  that  are  especially  characterized 
by  bitter  tastes  are  the  alkaloids.  These  include  such 
compounds  as  morphine,  cocaine,  pilocarpine,  quinine, 
nicotine,  and  strychnine,  the  bitterest  of  all  substances. 
In  aqueous  solution  these  substances  are  the  most  effec- 
tive agents  in  exciting  the  bitter  taste.  Gley  and  Richet 
(1885)  determined  that  strychnine  monochloride  could  be 
tasted  at  0.0006  gram  per  liter  of  water.  Of  such  a 
solution  5  cubic  centimeters,  which  was  the  volume  used 
by  these  investigators  in  their  individual  tests,  contains 
only  0.000005  gram  of  the  bitter  material  and  yet  this  very 
small  amount  produces  a  pronounced  taste.  Quinine 
hydrochloride  can  be  tasted  in  a  solution  as  dilute  as 
0.00004  molar  (Parker  and  Stabler,  1913).  Thus  bitter 
substances  far  exceed  hydrogen  ions  in  their  capacity  to 
stimulate  at  high  dilution. 

What  peculiar  chemical  feature  is  characteristic  of 
bitter  organic  substances  whereby  they  excite  this  taste 
is  at  best,  poorly  understood.  Henry  (1895)  pointed  out 
that  the  bitter  compounds  often  included  the  group 



and  this  was  confirmed  by  Cohn  (1914)  whose  extensive 
study  of  the  sapid  organic  compounds  led  him  to  the  con- 
clusion that  there  were  several  such  groups,  the  presence 


of  any  one  of  which  in  a  given  compound  would  give  it 
a  bitter  taste.  In  dyes  color-radicals  have  long  been 
called  chromophores ;  by  analogy  radicals  concerned  with 
taste  have  been  designated  saprophores.  Among  these 
are  hydroxyl  and  the  amine  group.  The  nitro  group  N02 
is  often  associated,  especially  in  aromatic  compounds, 
with  a  bitter  taste.  When  three  N02  groups  are  included 
in  a  given  compound,  it  always  has  a  bitter  taste ;  when 
two  are  at  hand,  the  taste  is  commonly  bitter  but  not 
invariably  so;  when  only  one  such  group  is  present,  the 
taste  is  not  bitter.  Thus  the  number  of  N02  groups  ap- 
pears to  be  significant  in  the  production  of  a  bitter  taste. 

The  bitter  taste,  then,  is  excited  by  several  classes  of 
substances;  by  ions  that,  with  the  possible  exception  of 
the  anion  of  picric  acid,  are  apparently  always  cations 
Herlitzka  (1908),  magnesium,  ammonium,  and  calcium; 
and  by  organic  substances,  especially  the  alkaloids,  which 
may  act  either  through  their  molecules  or  through  certain 
atomic  groups,  the  so-called  saprophores. 

7.  The  Sweet  Taste.  The  sweet  taste  is  excited  by 
the  diatomic  and  polyatomic  alcohols  of  the  aliphatic 
series,  by  the  aldehydes  and  ketons  derived  from  these 
alcohols,  and  especially  by  the  hexoses  whose  polymeriza- 
tion products,  the  disaccharides  arid  polysaccharides,  are 
in  this  respect  particularly  important.  Besides  these 
carbohydrates  other  organic  compounds,  such  as  chloro- 
form, dextro-asparagine,  and  saccharine,  have  sweet 
tastes.  Among  inorganic  substances  neutral  acetate  of 
lead,  often  called  sugar  of  lead,  and  the  salts  of  glucinum 
are  known  to  be  sweet.  Solutions  of  the  alkalis,  if  they 
are  of  appropriate  dilution,  are  said  likewise  to  excite 
this  taste. 


What  occasions  the  sweet  taste  of  lead  acetate  seems 
never  to  have  been  ascertained.  On  the  other  hand  glu- 
cinum  chloride  and  glucinum  sulphate,  both  of  which 
break  into  ions  in  water,  have  been  shown  by  Hober  and 
Kiesow  (1898)  to  owe  their  sweet  taste  to  their  common 
constituent,  the  glucinum  ion.  Thus  ions  are  one  means 
of  exciting  this  taste. 

But  the  sweet  taste,  like  the  bitter  one,  is  primarily 
associated  with  organic  compounds.  It  centers  about  the 
alcohols  and  especially  the  sugars  in  much  the  same  way 
that  the  bitter  taste  does  about  the  alkaloids.  Although 
the  halogenated  hydrocarbon  chloroform  and  the  aromatic 
compound  saccharine  are  both  sweet,  the  latter  about  500 
times  as  much  so  as  cane  sugar,  the  great  majority  of  sweet 
substances  are  aliphatic  alcohols  and  their  derivatives. 
Ethyl  alcohol  is  sweetish  in  taste  as  well  as  the  trihydric 
alcohol  glycerol,  but  the  type  of  sweet  substances  is  cane 
sugar  or  sucrose.  This  can  be  tasted  in  aqueous  solution 
to  about  0.02  molar;  in  weaker  concentrations  it  is  diffi- 
cult to  distinguish  it  from  pure  water.  Ethyl  alcohol 
cannot  be  tasted  in  solutions  much  weaker  than  3  molar,  a 
relatively  high  concentration  (Parker  and  Stabler,  1913). 

What  determines  the  sweet  taste  in  carbohydrates  is 
by  no  means  settled.  It  apparently  turns  upon  very 
slight  differences.  These  are  sometimes  sterioisomeric 
in  character.  Thus,  as  already  stated,  dextro-asparagine 
is  sweet  and  laevo-asparagine  is  tasteless.  Dextro-man- 
nose  is  sweet  and  its  stereoisomer  dextro-glucose  is  bitter. 
Other  such  examples  are  known.  In  some  instances 
slight  changes  in  composition  are  accompanied  by  con- 
siderable changes  in  taste.  Thus,  according  to  Thorns 
and  Nettesheim  (1920),  dulcin  loses  its  sweetening  power 


when  acidic  or  basic  substitutes  are  introduced  into  its 
benzene  nucleus.  The  introduction  into  a  sweet  molecule 
of  any  considerable  radical,  especially  an  aromatic 
one,  is  very  likely  to  be  followed  by  a  change  from 
sweet  to  bitter. 

Colin  (1914)  made  an  elaborate  comparison  of  the 
constitution  of  the  sweet  substances,  as  he  did  that  of 
the  bitter  compounds,  and  came  to  the  conclusion  that 
these  substances  like  the  bitter  ones  contained  particular 
groups  of  atoms  that  determined  their  taste  and  that  he 
designated  as  glucogenes.  Thus  among  alcohols  one  hy- 
droxyl  is  accompanied  with  slight  sweetness  and  four 
or  five  with  intense  sweetness.  But  notwithstanding 
the  extent  of  Colin 's  comparisons,  Oertly  and  Myers 
(1919)  found  his  generalizations  inadequate,  and  pro- 
posed in  place  of  his  hypothesis  one  in  which  two  groups 
were  assumed  to  be  present  in  every  sweet  molecule.  Fol- 
lowing by  analogy  the  terminology  used  for  dyes,  one  of 
these  groups  was  called  a  glucophore  and  the  other  an 
auxogluc.  By  a  close  comparison  of  the  sugars,  amino 
acids,  and  halogen  derivatives  of  the  hydrocarbons,  they 
believed  they  could  identify  at  least  six  glucophores  and 
nine  auxoglucs.  The  glucophores  are  (1)  CH2OH-CHOH-, 
(2)  -CO-CHOH-(H),  (3)  C02H-CHNH2-,  (4)  -CH2ON02, 

(5)  C§J-*-,  and  WC^-'-C^-.  The  auxoglucs  are  (1) 

H-,  (2)XCH,-,  (3)  CH3CH2-*  (4)  CH,-CH2-CH2-,  (5) 
(CH3)2CH-,  (6)  CH,OH-,  (7)  CH,CHOH-,  (8)  CH20!H- 
CH2-,  and  (9)  radicals  C^Ho^On  of  normal  polyhe- 
dric  alcohols. 

An  illustration  of  the  way  in  which  Oertly  and  Myers' 
theory  may  be  made  to  apply  to  sweet  substances  is  given 


in  the  following  table  in  which  the  resolution  of  a  number 
of  sweet  compounds  into  glucophores  and  auxoglucs 
is  indicated. 

Table  VII. 

A  table  of  sweet  organic  compounds  (aliphatic  series)  showing  the 
constitution  of  the  compound  and  its  resolution  into  a  glucophore  and  an 
auxogluc,  from  Oertly  and  Myers  (1919). 

Name  of 

Compound  Constitution  Glucophore  Auxogluc 

Glycol  CH2OH-CH2OH  CH2OH-CHOH  H- 


Fructose  CH2OHCO  ( CHOH ) 8CH2OH  .COCHOH-(H)  CnH2n+iOn- 

Glycine  CH2NH2-COOH  -CHNH2-COOH  H- 

Ethyl  nitrate  C2H3ON02  -CH2ON02  CH3- 

Notwithstanding  the  elaborate  attempts  of  Cohn  and 
of  Oertly  and  Myers  to  elucidate  the  chemoreception  of 
sweet  substances,  the  subject  must  be  admitted  to  be  one 
that  is  far  from  settled.  What  may  be  said  with  cer- 
tainty is  that  the  sweet  taste,  like  the  bitter  taste,  is  ex- 
cited both  by  ions  and  by  organic  molecules  the  details 
of  whose  activity,  however,  are  by  no  means  fully 
worked  out. 

8.  Inadequate  Stimuli.  Taste  is  somewhat  remark- 
able for  its  paucity  of  inadequate  stimuli.  Although  the 
tongue  is  very  sensitive  to  temperature  differences,  these 
changes  do  not  seem  to  excite  the  gustatory  receptors. 
It  is  questionable  whether  mechanical  stimulation,  such 
as  tapping  the  tongue  as  practised  by  the  older  physiolo- 
gists, will  call  forth  sensations  of  taste.  The  only  really 
effective  form  of  inadequate  stimulus  for  the  gustatory 
organs  seems  to  be  the  electric  current.  As  early  as 
1752  Sulzer  noted  the  peculiar  sensations  when  two  dif- 


ferent  metals  are  placed  simultaneously  on  the  tongue. 
This  observation  was  independently  made  by  Volta  in 
1792  who  believed  these  sensations  to  be  produced  by  the 
electrical  stimulation  of  the  organs  of  taste,  for  he  ob- 
tained the  same  effects  by  passing  an  electric  current 
through  the  tongue.  Five  years  later,  however,  Humboldt 
pointed  out  that  the  real  stimulating  agent  in  the 
so-called  electric  taste  might  be  the  substances  produced 
by  electrolysis  at  the  region  where  the  current  passes 
from  the  electrode  into  the  tongue  rather  than  the  electric 
current  itself.  Thus  was  established  the  two  opposing 
views  concerning  electrical  taste. 

If  an  electric  current  is  passed  through  the  human 
body  in  such  a  way  that  the  anode  is  applied  to  the  tongue 
and  the  cathode  to  some  other  part,  a  sour  taste  develops 
around  the  anode.  If  the  electrodes  are  reversed  in  posi- 
tion, an  alkaline  taste  appears  at  the  cathode.  This  con- 
forms with  what  takes  place  when  an  electric  current  is 
passed  through  an  alkaline  solution,  such  as  the  saliva; 
hydrogen  ions  appear  at  the  anode  and  hydroxyl  ions 
at  the  cathode.  Why  then  are  not  these  two  substances, 
the  hydrogen  and  the  hydroxyl,  the  stimuli  for  the  char- 
acteristic tastes? 

But  Rosenthal  (1860)  and,  before  him,  Volta,  found 
that  if  the  anode  is  a  weak  alkaline  solution  into  which 
the  tip  of  the  tongue  is  dipped,  a  sour  taste  nevertheless 
arises,  though  the  hydrogen  ions  under  such  a  combination 
might  be  expected  to  be  neutralized  immediately  by  the 
hydroxyl  present.  Rosenthal  also  showed  that  if  an  elec- 
tric current  is  passed  through  the  bodies  of  two  persons 
and  is  completed  by  bringing  the  tip  of  the  tongue  of  one 
of  these  individuals  into  contact  with  that  of  the  other, 


the  two  persons  experience  different  sensations,  one  sour 
and  the  other  alkaline.  These  and  other  like  experi- 
ments led  Eosenthal  to  conclude  that  the  electric  current 
itself  was  the  stimulating  agent  and  not  the  materials 
produced  by  electrolysis. 

But  it  must  not  be  forgotten  that  the  electrical  stimu- 
lation of  organs  of  taste  is  productive  of  a  variety  of 
sensations.  Thus  in  1798  Ritter  showed  that  after  a  cur- 
rent had  been  passing  for  some  time  through  the  tongue 
the  sour  taste  of  the  anode  changed  first  to  bitter  and  then 
to  alkaline  while  the  cathodic  alkaline  taste  changed  to 
sour.  Hofmann  and  Bunzel  (1897)  demonstrated  that 
during  the  passage  of  a  current  there  is  at  the  cathode 
a  burning  bitter  sensation  which  changes  to  a  sour  metal- 
lic taste  on  breaking  the  current.  The  initial  taste  they 
believed  to  be  due  to  the  products  of  electrolysis.  Von 
Zeynek  (1898)  also  accepted  this  explanation  for  the  elec- 
tric taste,  Gertz  (1919),  however,  pointed  out  that  the 
alternating  current  is  really  more  effective  in  exciting 
taste  than  the  direct  current  and  that  hence  the  electric 
taste  may  be  aroused  by  other  means  than  the  products 
of  electrolysis.  It  is  not  at  all  impossible  that  the  gusta- 
tory organs  may  be  excited  in  both  ways :  by  the  materials 
of  electrolytic  decomposition  and  directly  by  the  electric 
current.  But  how  an  electric  current  can  stimulate  gus- 
tation without  in  some  way  bringing  about  a  chemical 
change,  at  least  within  the  gustatory  cell,  is  difficult 
to  imagine. 

The  extreme  sensitiveness  of  the  organs  of  taste  to 
electrical  stimulation  is  not  only  characteristic  of  man, 
but  is  probably  found  throughout  the  vertebrates. 
Among  fishes  the  catfish  or  horned  pout,  Amiurus,  is  ap- 


parently  easily  stimulated  in  this  way.  The  head  and 
especially  the  eight  barbels  about  the  mouth  of  Amiurus 
are  richly  supplied  with  taste-buds.  These  organs,  like 
those  on  the  human  tongue,  are  apparently  extremely 
sensitive  to  metals  probably  because  of  the  slight  electric 
currents  produced  by  these  bodies,  for,  the  fishes  respond 
with  great  readiness  to  a  weak  constant  current  from  a 
dry  cell.  If  such  a  current  is  led  into  an  aquarium 
through  a  water-filled  glass  tube  and  out  again  by  a  sim- 
ilar tube,  the  water  acting  as  a  conductor,  catfishes  can 
be  readily  stimulated  by  bringing  such  tubes  close  to 
them.  If  the  current  is  sufficiently  reduced  (a  little  less 
than  a  microampere)  the  fishes  will  approach  the  open 
ends  of  the  tubes  and  nibble  at  the  current  as  though  it 
were  a  bait,  thus  giving  evidence  that  the  organs  stimu- 
lated are  the  gustatory  receptors  (Parker  and  Van 
Heusen,  1917).  Hence  the  electric  stimulus  seems  in  every 
way  to  duplicate  the  stimulus  normal  for  the  organ  of 
taste,  a  solution  of  a  sapid  substance. 

9.  Distribution  of  Tastes  on  the  Tongue.  The  four 
well-recognized  tastes,  as  the  preceding  sections  show, 
are  normally  excited  by  very  different  stimuli.  The  sour 
taste  is  dependent  upon  the  cation,  hydrogen.  The  saline 
taste  is  called  forth  by  a  number  of  anions :  chlorine, 
bromine,  iodine,  and  the  sulphate  and  nitrate  ions.  The 
bitter  taste  has  as  stimuli  the  alkaloids,  such  cations  as 
magnesium,  ammonium,  and  calcium,  and  possibly  the 
anion  of  picric  acid.  The  sweet  taste  depends  upon  such 
organic  compounds  as  the  sugars  and  alcohols,  and  on 
saccharine,  on  lead  acetate,  and  on  hydroxyl  and  gluci- 
num  ions.  The  four  tastes,  therefore,  are  excited  by 
entirely  independent  groups  of  stimuli  and  it  is  a  matter 



of  importance  to  ascertain  in  what  other  respects  they 
are  independent.  This  question  can  be  well  approached 
from  the  standpoint  of  their  distribution  on  the  tongue. 
As  already  mentioned,  the  tongue  of  the  normal  adult 
human  being  is  only  in  part  gustatory,  its  lower  surface 
and  the  central  portion  of  its  upper  surface  being 

A  B  C  D 

FIG.  36. — Diagrams  of  the  right  half  of  the  human  tongue  illustrating  the  distribution  of 
the  four  tastes;  the  dots  represent  the  area  and  concentration  of  a  given  taste:  A,  the  sour 
taste,  concentrated  on  the  edge;  B,  the  saline  taste,  concentrated  at  the  tip  and  on  the  edge; 
C,  the  bitter  taste,  concentrated  at  the  base;  D,  the  sweet  taste,  concentrated  at  the  tip. 
Modified  from  Hanig,  1901. 

devoid  of  taste.  This  sense  is  resident  only  on  the 
tip,  the  edges  and  the  dorsal  part  of  the  root  of  the 
tongue.  The  distribution  of  the  several  tastes  over  the 
gustatory  portion  of  the  tongue  has  been  a  matter  of 
investigation  for  physiologists  during  more  than  a  cen- 
tury, and  the  results,  particularly  among  the  recent 
workers,  have  been  remarkably  consistent  and  harmoni- 
ous. Shore  (1892),  Kiesow  (1894-1896),  and  Hanig 
(1901)  have  been  the  most  important  recent  contributors 


to  this  subject.  Their  work  shows  that  the  four  tastes 
have  decidedly  individual  distributions  on  the  tongue. 
The  sour  taste  is  best  developed  on  the  lateral  edges  of 
the  tongue  and  diminishes  from  these  regions  toward  the 
tip,  the  base,  and  the  central  anaesthetic  area  (Fig.  36,  A). 
The  saline  taste  is  most  pronounced  at  the  tip  and  on 
the  lateral  margins  of  the  tongue  and  diminishes  at  the 
base;  toward  the  central  area  it  ends  rather  abruptly 
(Fig.  36,  B).  The  bitter  taste  is  most  characteristic  of  the 
base  of  the  tongue  especially  in  the  region  of  the  vallate  pa- 
pillae whence  it  diminishes  rapidly  toward  the  central  area 
and  over  the  lateral  edges  to  the  tip  (Fig.  36,C).  The  sweet 
taste  is  at  its  maximum  at  the  tip  of  the  tongue  and  di- 
minishes thence  along  the  lateral  margins  to  the  base  (Fig. 
36,  D).  Thus  sour  is  represented  by  two  marginal  re- 
gions, saline  by  a  horse-shoe  shaped  area  at  the  tip,  bit- 
ter by  a  single  center  at  the  base,  and  sweet  by  one  at 
the  tip.  It  is  difficult  to  explain  these  differences  in  the 
distribution  of  the  tastes  except  on  the  assumption  of 
an  independent  sensory  mechanism  for  each  taste. 

This  interpretation  of  taste  is  strengthened  by  what 
has  been  learned  from  the  local  stimulation  of  the  tongue. 
Oehrwall  (1891)  mapped  out  a  group  of  fungiform  papillae 
near  the  tip  of  the  tongue  in  such  a  way  that  the 
individual  papillae  could  be  reidentified  and  studied. 
Each  papilla  was  stimulated  by  applying  to  it  the  point 
of  a  very  fine  brush  loaded  with  a  strong  solution  of  a 
given  substance.  The  substances  used  were  tartaric 
acid  2  per  cent,  common  salt  20  per  cent,  quinine  hydro- 
chloride  2  per  cent,  and  sugar  40  per  cent.  The  salt  was 
finally  abandoned  because  of  the  indistinctness  of  the 
sensation.  In  all  125  easily  identifiable  papillae  were 


tested.  All  of  these  were  found  to  be  sensitive  to  touch, 
warmth,  and  cold,  but  only  98  were  stimulated  by  the 
solutions  used.  The  results  of  these  tests  so  far  as  they 
relate  to  taste  are  given  in  the  following  tabulation. 

Test  Substances  Acid  Quinine  Sugar 

Number   of   papillse    sensitive 91  71  79 

Number  exclusively  sensitive! 12  0  3 

The  fact  that  12  papillae  were  stimulated  by  tartaric 
acid  but  not  by  quinine  or  sugar  and  that  3  were  stimu- 
lated by  sugar  but  not  by  tartaric  acid  or  quinine  is  strong 
evidence  in  favor  of  the  independence  of  at  least  the  sour 
and  sweet  tastes.  Oehrwall  also  discovered  by  his 
method  of  local  stimulation  that  the  surface  of  the  tongue 
between  the  papillae  was  insensitive  to  taste. 

This  result  confirmed  the  earlier  work  of  Goldscheider 
and  Schmidt  (1890)  who  had  also  shown  that  when  pa- 
pillae were  tested  with  a  mixed  solution  of  sugar  and 
quinine  sometimes  a  sweet  taste  was  evoked  and  at  other 
times  a  bitter  one. 

This  whole  subject  was  thoroughly  re-investigated  by 
Kiesow  (1898),  who  used  as  stimuli  solutions  of  hydro- 
chloric acid,  of  sodium  chloride,  of  quinine  sulphate  and 
of  sugar.  Of  the  39  papillae  tested  4  were  found  to  be 
insensitive.  The  conditions  presented  by  the  remaining 
35  are  summarized  in  the  following  tabulation. 

Test  Substances                                   Acid  Salt  Quinine  Sugar 

Number  of  papillae  sensitive 18  18                  13  26 

Number    exclusively    sensitive 3  3                   0  7 

Failed  of  stimulation 17  17                 22  9 

These  results  confirm  and  extend  the  original  findings 
of  Oehrwall  in  that  they  show  the  independence  of  the 
sour,  saline,  and  sweet  tastes.  The  fact  that  the  region 


tested  was  near  the  tip  of  the  tongue  is  probably  the 
occasion  of  the  absence  in  the  records  of  any  papillae 
stimulated  exclusively  by  quinine,  for  this  region  is  one 
in  which  the  bitter  taste  is  least  developed.  Kiesow  also 
observed  that  the  papillae  presented  a  great  variety  of 
combinations  in  taste ;  some  were  open  to  stimulation  by 
two  of  the  four  reagents  used,  others  by  three  and  still 
others  by  all  four.  It  is  known  that  each  gustatory 
papilla  carries  a  number  of  taste-buds  but  whether  in 
those  papillae  that  are  open  to  stimulation  by  two  or  more 
sapid  solutions  there  is  a  corresponding  number  of  kinds 
of  buds,  one  for  acid,  another  for  salt  reception  and  so 
forth,  cannot  be  stated,  for  it  is  possible  that  this  dif- 
ferentiation may  reach  to  the  gustatory  cells  of  each 
bud.  What  can  be  affirmed,  however,  is  that  in  those 
papillae  that  respond  exclusively  to  one  taste  all  taste- 
buds  with  their  contained  cells  must  be  so  constituted  as 
to  be  open  to  stimulation  by  one  class  of  sapid  substances 
and  to  be  closed  to  all  other  classes.  Thus  in  a  papilla 
that  is  stimulated  exclusively  by  acid  the  protoplasm  of 
the  receptive  cells  in  all  its  taste-buds  must  be  organized 
to  receive  acid  stimuli  and  not  to  react  to  those  for  the 
saline,  bitter  and  sweet  tastes.  This  conclusion  amounts 
to  a  very  complete  confirmation  of  Miiller's  theory  of  the 
specific  energy  of  sensory  nerves  as  it  is  interpreted  in 
modern  physiology  and  to  the  component  theory  as  ap- 
plied to  taste. 

10.  Action  of  Drugs  on  Taste.  Certain  drugs  have 
the  remarkable  property  of  temporarily  diminishing  or 
even  obliterating  taste.  Edgeworth  discovered  that  af- 
ter a  person  had  masticated  the  leaves  of  the  Indian 
asclepiad  Gymnema  sylvestre,  he  was  unable  to  taste 


sugar.  Hooper  (1887)  extracted  from  the  leaves  of  this 
plant  a  compound  that  he  named  gymnemic  acid  and  that 
he  showed  to  be  the  substance  that  affected  taste.  Ac- 
cording to  him  gymnemic  acid  tends  to  obliterate  the 
sweet  and  bitter  tastes  but  has  no  effect  on  the  saline  and 
sour  tastes.  Shore  (1892)  studied  the  influence  of  gym- 
nema  decoctions  on  the  tongue  and  found  that  they  oblit- 
erated the  sweet  taste  of  glycerine  very  easily  and  the 
bitter  taste  of  quinine  almost  as  readily.  They  had  very 
little  effect  on  the  taste  of  sulphuric  acid  or  of  common  salt. 
These  results  were  confirmed  in  the  main  by  Kiesow 
(1894).  Thus  gymnemic  acid  divides  the  tastes  into  at 
least  two  distinct  classes,  one  including  sweet  and  bitter, 
and  the  other  sour  and  saline. 

Stovaine  is  also  known  to  abolish  sweet  and  bitter 
without  obliterating  saline  and  sour  (Ponzo,  1909)  and 
eucaine-B  especially  reduces  bitter  (Fontana,  1902). 
Saline  and  sweet  tastes  and  in  less  degree  bitter  are 
reduced  by  a  0.02  normal  solution  of  chromium  nitrate 
(Herlitzka,  1909). 

The  effect  of  cocaine  on  taste  is  very  profound.  Von 
Anrep  (1880)  and  Knapp  (1884)  observed  that  this  nar- 
cotic was  capable  of  abolishing  completely  all  taste. 
Aducco  and  Mosso  (1886)  showed,  however,  that  it  acted 
more  energetically  on  the  bitter  taste  than  on  the  others. 
Shore  (1892)  found  that  on  treatment  with  cocaine  the 
buccal  sensations  were  extinguished  in  a  definite  order 
as  follows :  pain,  bitter,  sweet,  saline,  sour,  and  touch,  a 
sequence  confirmed  by  Kiesow  (1894).  Thus  cocaine  is 
more  selective  in  its  effect  on  taste  than  gymnemic  acid 
and  leads  to  a  separation  of  all  four  tastes. 

11.     Substances  with  two  Tastes.    A  number  of  sub- 


stances  are  known  that  possess  different  tastes  depend- 
ing upon  the  part  of  the  tongue  to  which  they  are  applied. 
Many  salts  have  this  peculiarity  Herlitzka  (1908). 
Potassium  nitrate  and  magnesium  sulphate  are  both  said 
to  be  saline  in  taste  when  applied  at  the  tip  of  the  tongue 
and  bitter  at  its  base.  This  action,  however,  is  proba- 
bly due  not  to  the  molecules  of  the  salts  but  to  their  ions. 
At  the  tip  of  the  tongue  the  anions  stimulate  the  organs 
of  the  saline  taste,  which  in  this  location  are  in  the  ascen- 
dency, and  at  the  base  of  the  tongue  the  cations  stimulate 
the  organs  of  the  bitter  taste  which  is  here  better  devel- 
oped. There  is  thus  a  kind  of  competition  between  the 
two  sets  of  ions,  as  Herlitzka  has  expressed  it,  and  in  one 
locality  the  anions  win  out,  in  the  other  the  cations. 

Such  an  explanation,  however,  does  not  apply  to  sub- 
stances like  parabrombenzoic  sulphinide.  This  material, 
according  to  Howell  and  Kastle  (1887)  has  a  distinctly 
sweet  taste  when  applied  to  the  tip  of  the  tongue  and  an 
intensely  bitter  one  at  the  back.  Dulcamarin,  the  gluco- 
side  from  bittersweet,  is  another  case  of  the  same  kind;  a 
list  of  these  is  given  by  Sternberg  (1898).  In  these  in- 
stances ions  are  probably  not  involved,  but  each  substance 
is  a  stimulus  for  both  the  organs  of  the  sweet  taste  and 
of  the  bitter  taste.  It  seems  impossible  to  explain  double 
tastes  such  as  those  just  mentioned  except  on  the  assump- 
tion of  independent  receptor  systems  for  the  tastes  con- 
cerned. Thus  far  no  substance  is  known  that  excites 
three  categories  of  tastes  though  I  know  of  no  reason  why 
such  a  substance  might  not  exist. 

12.  Latency  of  Taste  Sensations.  Von  "Wittich 
(1868)  appears  to  have  been  the  first  to  attempt  to  meas- 
ure the  interval  of  time  between  the  application  of  a 


stimulus  to  a  gustatory  portion  of  the  tongue  and  the 
response  of  the  subject.  He  used  an  electric  current  as  a 
stimulus  and  found  the  average  time  to  be  0.167  seconds. 
Von  Vintschgau  and  Honigschniied  (1875-1877),  who  used 
solutions  of  various  substances  as  stimuli,  found  that  the 
times  were  different  for  the  different  tastes,  being  short- 
est for  saline,  longer  for  sweet,  still  longer  for  sour  and 
longest  for  bitter.  They  also  discovered  that  the  times 
were  different  for  the  tip  of  the  tongue  and  its  base. 
Their  results  were  confirmed  in  general  by  the  later  in- 
vestigations of  Beaunis  (1884),  of  Henry  (1895)  and  of 
Kiesow  (1903)  who  recorded  the  following  periods  for 
the  tip  of  the  tongue : 

Sodium    chloride 0.308  second 

Sugar 0,446  second 

Hydrochloric  acid 0.536  second 

Quinine 1.082  second 

These  records  agree  with  Schirmer's  observation 
(1859)  that  when  a  solution  containing  all  four  sapid  sub- 
stances is  placed  on  the  tongue,  the  subject  experiences 
the  sensations  in  the  order  saline,  sweet,  sour,  and  bitter. 
They  also  confirm  the  opinion  that  the  four  tastes  are 
separate  entities. 

One  aspect  of  the  problem  of  gustatory  latency  turns 
on  temperature.  If  the  stimulation  of  a  taste  receptor 
is  a  chemical  operation,  this  process  should  exhibit  a  con- 
siderable temperature  co-efficient  that  might  make  itself 
felt  in  a  change  in  the  latent  period.  But  so  far  as  I  am 
aware  no  studies  with  this  point  in  view  have  been  car- 
ried out. 

13.     Taste  Alterations;  After-tastes.     A  number  of 


substances  are  known  whose  solutions  so  affect  the  tongue 
that  its  powers  of  taste  become  temporarily  changed. 
Thus  these  substances  give  rise  to  what  have  been  called 
after-tastes.  In  almost  every  instance  the  taste  that 
suffers  change  is  the  sweet  taste  and  this  is  increased  in 
efficiency.  Thus  Aducco  and  Mosso  (1886)  found  that 
after  the  tongue  had  been  held  in  dilute  sulphuric  acid 
for  five  to  ten  minutes,  distilled  water  was  then  capable 
of  exciting  a  very  sweet  taste.  A  solution  of  quinine  was 
also  sweet  to  the  taste  at  the  tip  of  the  tongue,  but  it 
remained  normally  bitter  at  the  base.  This  change  was 
not  brought  about  by  other  acids  such  as  acetic,  citric,  and 
formic.  Frentzel  (1896)  also  noticed  that  after  washing 
out  the  mouth  with  a  weak  solution  of  copper  sulphate, 
smoking  a  cigar  was  accompanied  by  a  sweet  taste.  Ac- 
cording to  Zuntz  (1892)  a  solution  of  sodium  chloride  of 
one  per  cent  strength  will  increase  the  sweetness  of  sugar, 
an  observation  confirmed  by  Heymans  (1899).  A  mouth 
wash  of  potassium  chlorate  is  well  known,  to  leave  the 
tongue  so  that  distilled  water  tastes  sweet  (Nagel,  1896). 
In  all  these  instances  it  is  probable  that  the  constitution 
of  the  receptor  for  the  sweet  taste  is  so  changed  by  the 
first  solution  applied  to  it  that  it  becomes  hypersensitive 
to  its  normal  stimuli  such  as  sugar  or  even  open  to  novel 
stimuli  such  as  distilled  water. 

Complete  loss  of  taste  or  ageusia  is  known  to  accom- 
pany hysterical  and  other  abnormal  nervous  states.  It 
may  be  temporary  or,  in  the  case  of  certain  lesions,  per- 
manent in  character. 

14.  Gustatory  Contrasts.  Although  some  acids  in- 
crease the  sensitiveness  of  the  sweet  taste  and  thus  give 
ground  for  a  gustatory  contrast,  it  is  questionable  whether 


such  contrasts  exist  as  extensively  as  was  believed  by  the 
older  workers.  It  is  a  common  opinion  that  after  a  sweet 
drink  a  sour  taste  is  more  intense,  but  Oehrwall  (1891) 
was  unable  to  confirm  this  experimentally  nor  could  he 
show  that  bitter  increased  the  sensibility  to  sweet. 
Haycraft  (1900)  noted  that  when  one  border  of  the  tongue 
is  rubbed  with  salt,  the  other  border  becomes  hypersensi- 
tive to  sugar,  but  such  a  contrast  is  clearly  not  peripheral 
but  central  in  origin,  and  possibly  other  contrasts  may 
be  thus  explained. 

15.     Taste  Compensations  and  Mixtures.     Mixtures 
of  sapid  solutions  do  not  as  a  rule  give  rise  to  tastes  other 
than  those  of  their  components.     Lemonade  has  both  the 
sweet  taste  of  the  sugar  and  the  sour  taste  of  the  citric 
acid  it  contains.     Sugar  adds  a  pleasant  element  to  cof- 
fee, but  does  not  destroy  its  bitter  taste.    In  ordinary 
food  the  flavor  is  the  mixture  of  true  tastes  and  odors 
accompanied  by  the  multitude  of  other  buccal  sensitivities 
due  to  the  variety  of  substances  in  the  mouth  and  accep- 
ted in  a  rather  unanalyzed  form  by  the  central  apparatus. 
Yet  in  all  this  complexity  the  elements  remain  essentially 
distinct.     Competition  rather  than  compensation  seems 
to  be  the  rule.     Kiesow  (1894-1896) ,  however,  has  claimed 
that  a  very  weak  solution  of  sugar  and  salt  gives  a  taste 
that  is  neither  sweet  nor  saline  but  distinctly  flat,  and 
Kremer   (1918)   has  recently  shown  that  a  solution  of 
sodium  chloride  too  weak  to  stimulate  the  saline  taste 
will,  nevertheless,  considerably  increase  the  sweetness  of 
a   cane-sugar   solution.     Quinine   hydrochloride   on  the 
other  hand  will,  according  to  Kremer,  reduce  sweetness. 
These  instances  may  be  evidence  of  gustatory  compensa- 
tion, but  it  seems  much  more  probable,  as  was  indicated 


in  a  preceding  section,  that  they  result  from  a  sensitizing 
or  a  desensitizing  of  the  sweet  receptors  by  the  sodium 
chloride  or  the  quinine,  for  it  is  extremely  doubtful,  as 
Oehrwall  (1891)  has  stated,  whether  true  gustatory  com- 
pensation ever  occurs.  Ionic  antagonism  such  as  Crozier 
(1915)  has  discovered  in  the  reaction  of  the  frog's  foot  to 
salt  solution  has  thus  far  not  been  identified  in  taste. 

16.  The  Gustatory  Senses.  When  a  general  survey 
of  the  so-called  sense  of  taste  is  made,  the  most  striking 
feature  that  appears  is  the  remarkable  independence  of 
the  four  categories,  sour,  saline,  bitter,  and  sweet.  These 
are  excited  by  groups  of  different  stimuli,  they  give  re- 
markable evidence  of  having  separate  receptors,  they  are 
differently  acted  upon  by  various  drugs,  and  they  show 
numerous  other  peculiarities  that  are  interpretable  only 
from  the  standpoint  of  organic  separateness.  So  im- 
pressed was  Oehrwall  (1891,  1901)  with  these  peculiari- 
ties that  he  declared  them  to  be  in  all  essentials  four 
separate  senses,  a  declaration  entirely  in  accord  with  the 
component  theory  as  applied  to  taste.  Although  this 
view  has  a  certain  radical  element  in  it  and  has  not  been 
favorably  received  by  such  workers  as  Kiesow,  Nagel, 
Luciani,  and  Henning,  who  have  declared  for  the  unitary 
nature  of  taste,  it  is  difficult  to  say  why  it  should  not  pre- 
vail. It  has  been  urged  that  gustatory  compensation  is 
inconsistent  with  Oehrwall 's  hypothesis  and  possibly  this 
may  be  true.  But  gustatory  compensation  is  so  uncer- 
tain a  phenomenon  that  when  compared  with  the  sub- 
stantial body  of  evidence  in  favor  of  the  hypothesis,  this 
objection  lacks  force.  Henning  (1916)  has  declared  that 
the  tastes  of  different  substances,  members  of  one  cate- 
gory, are  not  necessarily  alike;  thus  the  saline  tastes  of 


sodium  chloride,  sodium  iodide,  and  sodium  bromide, 
though  much  the  same  are  still  characteristically  different. 
And  he  has  further  maintained  that  the  mixed  tastes  so- 
called  cannot  be  imitated  by  real  mixtures;  thus  the 
bitter-saline  taste  of  magnesium  chloride  cannot  be  repro- 
duced, he  believes,  by  a  mixture  of  sodium  chloride  and 
bitter  aloes.  But  all  such  statements  imply  that  the 
conception  of  the  receptive  independence  of  tastes  neces- 
sarily involves  the  further  view  that  a  gustatory  stimulus 
is  limited  to  one  category  of  receptors.  That  some  sub- 
stances, such  as  parabrombenzoic  sulphinide,  stimulate 
two  categories  of  receptors  has  already  been  made  clear 
and  though  most  stimulating  materials  influence  in  a 
vigorous  way  only  one  set  of  end-organs,  it  is  more  than 
probable  that  they  all  affect  at  least  to  a  slight  degree 
other  such  sets.  The  taste  of  any  substance  then  is  not 
necessarily  one  of  the  four  tastes  and  this  alone,  but  one 
of  these  qualified  by  traces  of  other  tastes  excited  slightly 
and  simultaneously  by  the  same  stimulating  agent. 
Hence  any  substance  such  as  sodium  chloride,  or  sodium 
bromide,  may  perfectly  well  have  a  somewhat  individual 
taste  without  doing  violence  to  the  hypothesis  that  there 
are  four  separate  tastes,  and  the  success  with  which 
mixed  tastes  so-called  may  be  imitated  is  rather  a  matter 
of  skill  than  despair. 

It  is  time  that  gustation  is  a  strikingly  unified  oper- 
ation, but  when  this  unity  is  looked  into,  it  is  seen  to 
depend  upon  simultaneousness  of  action  rather  than  on 
interdependence  of  activities.  Smell  is  related  to  taste 
in  much  the  same  way  that  one  taste  is  related  to  another. 
On  the  whole  it  would  seem  more  consistent  with  fact  to 
speak  of  the  sour  sense,  the  saline,  the  sweet,  and  the 


bitter  sense  than  of  the  sense  of  taste.  Just  as  the  sense 
of  feeling  in  the  skin  has  been  shown  to  consist  of  at 
least  four  senses,  touch,  pain,  heat,  and  cold,  so  taste  must 
be  regarded  as  composed  of  at  least  four  senses.  That 
these  act  together  and  in  everyday  experience  produce  a 
unified  effect  upon  us  is  no  more  reason  for  classing  them 
as  one  sense  than  in  the  case  of  the  integumentary  senses. 
The  sense  of  taste  must,  therefore,  be  regarded  as  a  ge- 
neric term  under  which  at  least  four  true  senses  are  gath- 
ered: sour,  saline,  bitter,  and  sweet  (Oehrwall,  1891, 1901). 

Although  the  sense  of  taste  thus  loses  a  certain  amount 
of  its  reality,  the  senses  classed  under  it  probably  possess 
a  kind  of  genetic  unity  that  is  not  without  significance. 
It  is  very  probable  that  these  four  senses  represent  four 
lines  of  differentiation  that  have  evolved  from  a  single 
ancestral  sense.  The  remarkable  uniformity  of  their 
structure  is  suggestive  of  this  view.  If  the  four  senses 
under  discussion  have  had  some  such  origin  as  that  just 
indicated,  the  term  sense  of  taste  might  well  apply  to 
that  primitive  state,  perhaps  represented  in  some  of  the 
lower  vertebrates  today,  from  which  the  four  gustatory 
senses  of  man  have  been  derived. 

17.  Comparative.  The  comparative  physiology  of 
taste  in  vertebrates  is  almost  an  untouched  field.  The 
distribution  of  taste-buds  in  the  vertebrate  classes  indi- 
cates the  presence  of  this  sense  in  the  mouth  regions  in 
forms  as  low  as  the  amphibians.  In  fishes  Herrick  ( 1903) 
lists  over  thirty -five  species  in  which  taste-buds  are  known 
to  occur  on  the  outer  surface  of  the  animal  as  well  as 
in  the  mouth.  The  catfish  Amiurus  is  remarkable  in  this 
respect  in  that  its  whole  outer  surface  is  provided  with 
these  organs  which  are  most  abundantly  present  on  the 


barbels.  When  a  piece  of  meat  is  brought  into  contact 
with  the  barbel  of  one  of  these  fishes,  the  animal  will 
immediately  seize  and  swallow  the  morsel.  The  same  is 
true  when  the  meat  is  brought  in  contact  with  the  side  of 
the  fish.  This  quick  seizure  and  swallowing  of  the  food 
has  been  called  by  Herrick  the  gustatory  response.  If 
a  barbel  or  the  flank  of  Amiurus  is  touched  with  a  pledget 
of  cotton  instead  of  the  meat,  the  fish  will  turn  toward 
the  object,  but,  as  a  rule,  will  not  snap  at  it.  This  Herrick 
has  designated  the  tactile  response.  If,  now,  the 
cotton  is  soaked  with  meat  juice  and  brought  to  the  side 
of  the  fish,  the  quick  gustatory  response  follows.  The 
same  form  of  response  is  made  to  meat  juice  discharged 
from  a  pipette  on  the  side  of  the  fish.  From  this  and 
other  tests  Herrick  concluded  that  the  gustatory  response 
in  Amiurus  could  be  called  forth  by  purely  gustatory 
stimuli  unaccompanied  by  touch  and  that  for  this  fish 
taste  is  accompanied  by  a  local  sign  as  touch  is.  That 
these  responses  are  really  gustatory  is  shown  by  the  fact 
that  when  the  branch  of  the  seventh  nerve  that  innervates 
the  taste-buds  on  the  flank  of  Amiurus  is  cut,  the  re- 
sponses no  longer  occur  (Parker,  1912). 

Conditions  similar  to  those  in  Amiurus  were  recorded 
by  Herrick  in  a  number  of  gadoid  fishes  and  it  is  thus 
clear  that  taste  is  a  general  integumentary  function  in 
many  of  these  animals.  To  what  extent  the  taste-buds  of 
the  fish  skin  are  differentiated  for  the  several  senses  of 
sour,  saline,  bitter,  and  sweet  cannot  be  stated.  It  is 
remarkable,  however,  that  in  almost  all  the  fishes  tested 
no  response  to  sugar  has  been  found  not  only  on  the  sur- 
face of  the  body  but  also  in  the  mouth  (Parker,  1912). 


The  sweet  sense  may,  therefore,  be  an  exclusive  posses- 
sion of  the  higher  vertebrates. 


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Contents. — 1.  Common  Features  in  the  Stimulation  of 
Chemoreceptors.  2.  Differences  among  Chemoreceptors. 
3.  Groups  of  Chemoreceptors.  4.  Classification  of  Recep- 
tors in  General.  5.  Genetic  Relations  of  Chemoreceptors. 
6.  Bibliography. 

1.  COMMON  Features  in  the  Stimulation  of  Chemo- 
receptors. The  sense  organs  that  have  been  discussed  in 
this  volume,  the  olfactory  organs,  the  vomero-nasal  or- 
gans, the  common  chemical  receptors,  and  the  organs  of 
taste,  form  a  more  or  less  natural  group  of  organs  under 
the  general  title  of  chemical  receptors  or  Chemoreceptors. 
This  designation  is  justified  by  the  fact  that  in  stimula- 
tion these  several  types  of  receptors  present  certain 
important  features  in  common.  In  all  instances  they 
are  activated  by  solutions.  This  is  most  obvious  in  the 
sense  of  taste  whose  stimuli  from  fishes  to  mammals  con- 
sist of  materials  in  solution  either  in  the  water  that  enters 
the  mouth  or  in  the  saliva  that  is  mingled  with  the  crushed 
food.  An  aqueous  solution  is  also  the  stimulus  for  the 
common  chemical  receptors.  The  nasal  cavities  of  fishes 
are  likewise  bathed  by  a  continuous  stream  of  water  that 
carries  the  stimulating  substances  to  the  olfactory  sur- 
faces. And  in  the  air-inhabiting  vertebrates,  as  already 
pointed  out,  the  olfactory  terminals  are  probably  not 
exposed  in  any  direct  way  to  the  air  that  carries  the  stim- 
ulating material  but  are  immersed  in  mucous  through 



which  this  material  must  make  its  way  before  it  becomes 
effective.  In  olfaction,  moreover,  it  is  probably  not 
simply  a  question  of  aqueous  solution  but,  as  already  ex- 
plained, one  of  solution  in  oil  as  well,  for  the  olfactory 
stimulus  seems  to  be  a  material  that  must  reach  its  recep- 
tors through  an  aqueous  medium  that  covers  them  and 
then  enter  them  through  their  lipoid  components.  What 
has  been  said  of  the  stimulation  of  the  olfactory  organ 
is  probably  true  of  the  vomero-nasal  organ  also.  Thus 
in  one  way  or  another  all  appropriate  stimuli  of  the  so- 
called  chemoreceptors  are  materials  in  solution. 

But  not  all  soluble  materials  stimulate  the  chemorecep- 
tors. Thus  such  elementary  gases  as  hydrogen,  oxygen, 
and  nitrogen  are  odorless  and  tasteless,  and  a  number  of 
organic  substances  have  no  stimulating  capacity  for 
these  organs.  Those  substances  that  do  stimulate,  as 
was  pointed  out  especially  in  the  case  of  taste,  fall  into 
groups  whose  characteristics  are  chemical  and  not  phys- 
ical and,  though  such  an  analysis  cannot  at  present  be 
made  with  certainty  for  smell,  it  has  already  been  pointed 
out  that  the  variety  of  smells  can  be  explained  only  on  a 
chemical  basis.  Thus  chemoreceptors  are  stimulated  not 
simply  by  material  in  solution,  but  by  the  chemical  activ- 
ity of  dissolved  material.  On  this  assumption  it  is  nat- 
ural to  expect  that  there  would  be  a  certain  number  of 
substances,  chemically  inert  toward  the  given  receptors, 
that  would,  therefore,  be  incapable  of  acting  as  stimuli 
for  them.  Such  substances  as  the  gases  already  men- 
tioned probably  represent  this  group. 

The  stimulus  for  the  chemoreceptor,  however,  is  not 
only  a  solution  of  a  chemically  active  material,  but  it  is 
such  a  solution  applied  directly  to  the  terminal  organ. 


This  peculiarity  of  the  chemoreceptors  is  in  strong  con- 
trast with  that  which  occurs  in  the  so-called  mechanicore- 
ceptors,  the  organs  of  touch,  pressure,  and  hearing.  In 
these  organs  the  appropriate  stimulus  is  a  deforming 
pressure  which  may  be  exerted  by  an  impinging  or  vi- 
brating material  that  does  not  necessarily  touch  the 
terminal  organ  itself,  but  may  act  through  a  considerable 
amount  of  intervening  tissue.  Hence  the  mechanicore- 
ceptors  are  not  necessarily  exposed  directly  to  what  is 
ordinarily  called  the  stimulus  as  chemoreceptors  are,  but 
they  may  be  excited  more  or  less  indirectly.  Our  organs 
of  touch  and  of  hearing,  therefore,  may  be  lodged  in  the 
deeper  part  of  the  skin  or  the  head  without  interfering 
in  any  serious  way  with  their  efficiency.  All  chemorecep- 
tors on  the  other  hand  are  necessarily  either  upon  the 
exposed  surfaces  of  the  body  or  are  provided  with  pores 
that  lead  from  these  surfaces  directly  to  the  receptors 
themselves.  This  condition  is  in  a  way  merely  a  corollary 
of  what  has  already  been  stated  about  chemical  stimula- 
tion, for  if  the  organs  of  smell,  taste  and  the  like  are 
acted  on  chemically  by  their  appropriate  stimuli,  these 
stimuli  must  of  necessity  come  into  direct  contact  with 
the  given  terminals. 

2.  Differences  among  Chemoreceptors.  The  chemore- 
ceptors agree  then  in  the  general  character  of  their  stim- 
uli. Such  stimuli  are  certain  chemically  active  materials 
in  solution  applied  directly  to  the  receptors  themselves. 
The  variety  that  these  organs  exhibit  ought,  therefore, 
to  turn  more  or  less  on  the  extent  of  their  differentiation 
in  relation  to  the  chemical  diversity  of  the  environment. 
The  degree  of  this  organic  differentiation,  however,  has 
been  very  inadequately  worked  out.  Almost  nothing  is 


known  of  the  stimuli  for  the  vomero-nasal  organ,  and 
very  little  has  been  done  on  those  for  the  common  chemi- 
cal sense.  The  senses  of  smell  and  of  taste  are  naturally 
much  better  known.  When  their  stimuli  are  compared 
they  are  found  in  general  to  belong  to  different  categories 
of  material;  what  is  smelled  is  generally  not  tasted  and 
what  is  tasted  is  not  smelled. 

These  two  categories  of  substances  afford  an  impor- 
tant basis  for  comparing  taste  and  smell.  This  can  be  done 
from  the  standpoint  of  the  minimum  concentrations  of 
materials  that  serve  as  stimuli  for  the  two  sets  of  recep- 
tors. Bitter  substances  are  apparently  the  most  effec- 
tive stimuli  for  the  sense  of  taste.  Quinine  hydrochloridc 
can  be  tasted  in  a  solution  as  weak  as  0.00004  molar, 
but  this  threshold  is  exceeded  by  that  of  what  is  probably 
the  most  bitter  of  all  substances  strychnine.  According 
to  Gley  and  Richet  (1885)  the  weakest  solution  in  which 
the  bitter  taste  of  strychnine  hydrochloride  can  be  distin- 
guished contains  only  0.0004  gram  of  this  substance  in 
one  liter  of  water.  This  is  approximately  equivalent  to 
one  and  a  half  million ths  of  a  molar  solution  (1.48xlO~6 
molar),  and  much  exceeds  in  this  respect  the  efficiency 
of  quinine.  One  of  the  strongest  odors  known  is  that  of 
mercaptan  of  which  according  to  Fischer  and  Penzoldt 
(1886),  0.01  milligram  evaporated  in  230  cubic  meters  of 
air  gives  a  perceptible  smell.  Assuming  the  substance 
used  by  these  investigators  to  have  been  methyl  mercap- 
tan, such  a  dilution  would  bo  represented  by  about  a 
million-millionths  molar  solution  (9xlOKi)  or  approxi- 
mately one  and  a  half  million  times  more  dilute  than  the 
weakest  solution  of  strychnine  that  can  be  tasted.  Thus 
the  olfactory  receptor  is  open  to  stimulation  by  a  very 


much  weaker  concentration  than  the  gustatory  one  is. 
It  might  be  maintained,  however,  that  the  line  of  ar- 
gument used  in  the  last  paragraph  is  invalid  because  it 
is  based  upon  measurements  of  one  substance  for  taste 
and  another  for  smell,  and  that,  therefore,  the  two  sets 
of  figures  are  not  fairly  comparable.  But  the  conclusion 
just  reached  is  also  supported  by  determinations  made 
with  a  single  substance.  Ethyl  alcohol  is  soluble  in  both 
water  and  oil  and  is  one  of  the  relatively  few  substances 
that  has  at  once  both  taste  and  smell.  As  a  matter  of 
fact  it  is  also  a  stimulus  for  the  common  chemical  sense. 
Hence  it  may  be  conveniently  employed  for  comparing  all 
three  classes  of  receptors.  When  such  a  test  is  made, 
it  is  found  that  the  weakest  concentration  of  alcohol  vapor 
that  can  be  smelled  is  about  0.000125  molar  and  that  the 
weakest  aqueous  solution  of  this  substance  that  can  be 
tasted  is  3  molar.  To  stimulate  the  common  chemical 
sense  with  ethyl  alcohol  requires  an  aqueous  solution  of 
strength  5  to  10  molar.  Hence  so  far  as  ethyl  alcohol 
is  concerned  smell  may  be  said  to  be  about  24,000  times 
more  delicate  than  taste  and  about  60,000  times  more 
delicate  than  the  common  chemical  sense.  From  the 
standpoint  of  a  single  substance  then,  smell  must  be  ad- 
mitted to  be  vastly  more  efficient  than  either  taste  or 
the  common  chemical  sense  both  of  which  lie  in  this  re- 
spect close  together  (Parker  and  Stabler,  1913).  Unfor- 
tunately the  stimulation  of  the  vomero-nasal  organ  has 
not  yet  been  studied  so  that  its  capability  from  this 
standpoint  is  not  known,  but,  judged  from  its  structure,  it 
probably  has  a  receptive  efficiency  not  far  from  that  of 
the  olfactory  organ.  In  that  case  the  chemoreceptors  of 
vertebrates  would  fall  into  two  groups,  the  olfactory  and 


voniero-nasal  organs  with  high  efficiency  and  the  common 
chemical  receptors  and  organs  of  taste  with  relatively 
low  efficiency.  These  two  sets  of  organs  might  in  this 
respect  be  compared  with  scales,  the  organs  of  taste  and 
of  the  common  chemical  sense  resembling  ordinary  scales 
on  which  only  gross  amounts  are  weighed  and  the  organs 
of  smell  and  the  vomero-nasal  organs  resembling  chemi- 
cal balances  on  \vhich  small  weights  may  be  determined. 

As  olfaction  deals  effectively  with  very  minute 
amounts  of  substance  and  gustation  only  with  much 
greater  amounts,  it  follows  that  materials  that  have  be- 
come highly  attenuated  by  being  broadly  spread  from 
their  sources  either  in  water  or  in  air  may  nevertheless 
still  be  concentrated  enough  to  stimulate  the  organs  of 
smell  though  they  can  have  no  possible  effect  upon  those 
of  taste.  Such  faint  odors  are  the  means  whereby  ani- 
mals scent  their  food,  find  their  mates,  or  avoid  their 
enemies.  Hence  the  olfactory  organ  has  been  appropri- 
ately classed  as  a  distance  receptor  or  exteroceptor,  to 
use  a  convenient  term  from  Sherrington  (1906),  in  that 
the  impulses  to  which  it  gives  rise  commonly  direct  the 
animal  toward  distant  points  or  away  from  them. 

Taste  and,  in  the  higher  vertebrates  at  least,  the  com- 
mon chemical  sense  are  stimulated  only  by  relatively  con- 
centrated solutions  such  as  occur  in  connection  with  the 
food.  Hence  the  responses  that  these  organs  call  forth 
are  concerned  with  the  swallowing  of  food,  with  the  re- 
jection of  material  taken  into  the  mouth,  with  mastication 
and  saliva  and  the  like.  These  receptors  are,  therefore, 
rightly  classed  as  interoceptors  though  it  must  be  re- 
membered, as  Herrick  (1918)  has  pointed  out,  that  in 
some  fishes,  such  as  the  catfishes,  taste-buds  serve  in  the 


discovery  of  food  as  well  as  in  its  appropriation,  and 
partake,  therefore,  more  or  less  of  the  nature  of  extero- 
ceptors.  Although  olfaction  has  a  function  independent 
and  separate  from  that  of  gustation  in  scenting  mates 
or  enemies  and  gustation  has  a  function  independent  of 
olfaction  initiates  the  feeding  reflexes  both  muscular  and 
noxious  material,  both  senses  are  intimately  associated 
in  feeding.  Food  is  found  and  the  digestive  secretions 
are  started  through  smell;  it  is  swallowed  and  these  se- 
cretions are  intensified  ordinarily  through  taste.  Thus 
olfaction  initiates  the  feeding  reflexes  both  muscular  and 
secretory  and  gustation  reinforces  and  completes  them. 
It  is  remarkable  that  in  some  fishes  like  the  catfishes 
(Amiurus)  and  especially  the  dogfishes  (Mustelus; 
Parker,  1914)  feeding  scarcely  ever  occurs,  even  when  the 
fishes  are  starving  and  food  is  present,  unless  the  process 
is  initiated  through  olfactory  reflexes.  These  seem  to 
be  essential  for  that  chain  of  events  that  result  in  the 
final  swallowing  of  the  food,  a  condition  that  shows  how 
intimately  smell  and  taste  are  interwoven  in  the  verte- 
brate organization. 

Smell  and  taste,  though  thus  most  closely  involved  in 
the  feeding  reflexes,  are  nevertheless  perfectly  distinct. 
As  long  ago  as  1821  Cloquet  (Larguier  des  Bancels,  1912) 
showed  that  on  closing  the  nose  by  pinching  the  nostrils 
smell  can  be  eliminated  and  only  taste  remains.  Under 
such  circumstances  it  is  surprising  to  those  who  have 
not  previously  tried  the  experiment  to  discover  how  small 
a  proportion  of  our  food  sensations  are  due  to  taste  and 
how  large  a  one  to  smell.  A  cold  in  the  head  commonly 
eliminates  smell  and  leaves  taste.  It  reduces  a  person 
to  a  state  in  which  food  is  often  described  as  without 


flavor,  for  only  sour,  saline,  sweet,  and  bitter  tastes  can 
be  sensed  and  onion  produces  the  same  sweetish  taste 
that  apple  does.  The  separateness  of  smell  and  taste 
depends  doubtless  upon  the  conditions  already  described. 
Smell  is  excited  in  general  by  one  set  of  substances ;  taste 
by  another.  Smell  calls  for  only  very  weak  solutions; 
taste  requires  relatively  strong  ones.  It  may  also  be  that 
these  two  senses  differ  in  the  nature  of  the  solutions  that 
activate  them;  taste  is  attuned  to  substances  that  form 
aqueous  solutions,  smell  to  those  that  dissolve  in  oil. 
Cell  surfaces  are  commonly  believed  to  be  diphasic  in 
that  they  are  composed  of  a  mixture  of  two  materials  one 
oily  and  the  other  aqueous.  The  gustatory  hairs  may  be 
so  constituted  that  the  aqueous  constituent  is  the  avenue 
of  entrance  for  the  stimulating  substance  and  the  olfac- 
tory hairs  so  that  the  oily  one  is  the  inlet.  If  such  is  the 
case,  this  feature  may  also  be  an  important  difference 
between  smell  and  taste. 

3.  Groups  of  Chemical  Receptors.  Taste  and  smell 
are  two  of  the  five  senses  ordinarily  attributed  to  man. 
But  in  the  detailed  study  of  the  human  senses  not  one 
has  escaped  a  kind  of  functional  subdivision  whereby  it 
has  been  shown  to  be  more  than  a  single  sense.  Thus  the 
internal  ear  originally  regarded  by  physiologists  as 
purely  an  organ  of  hearing,  was  shown  by  Flourens  in 
1828  to  be  concerned  in  a  most  important  way  with  bodily 
equilibrium.  From  this  standpoint  the  ear  takes  on  the 
character  of  a  double  sense  organ.  This  duplicity  is 
especially  well  marked  in  certain  fishes  in  which  the 
membranous  labyrinth  is  completely  divided  in  two  cor- 
responding to  the  functional  differentiation  already  in- 
dicated; one  of  these  parts  consists  of  the  utriculus  with 


its  three  semicircular  canals  and  has  to  do  with  equi- 
librium and  the  other  of  the  sacculus  and  its  appended 
lagena  and  is  concerned  with  hearing.  Even  so  unified 
an  organ  as  the  human  eye  is  made  up  of  an  intermingling 
of  two  receptive  fields,  for,  as  originally  suggested  by 
Schultze  (1866)  and  as  elaborated  by  von  Kries  (1904), 
the  retinal  rods  are  concerned  with  colorless  vision  in 
dim  light  and  the  cones  with  color  vision  in  bright  light. 
Thus  the  eye  is  differentiated  for  two  kinds  of  sight,  one 
by  night  and  the  other  by  day.  The  integumentary  sense 
originally  supposed  to  be  unitary,  was  shown  by  Blix  in 
1884  to  consist  of  at  least  three  senses,  cold,  warm,  and 
pressure.  To  these  were  added  in  1896  by  von  Frey  a 
fourth,  pain.  Thus  it  is  clear  that  the  conception  of  five 
senses  for  man  is  wholly  inadequate  and  though  numbers 
are  perhaps  not  the  best  way  of  indicating  the  sensory 
equipment  of  human  beings  or  in  fact  of  any  other  ani- 
mal, it  is  not  without  interest  to  record  the  opinion  of 
Herrick  (1918)  that  the  classes  of  human  receptors  are 
now  known  to  be  more  than  twenty. 

The  chemoreceptors,  represented  in  the  older  accounts 
by  the  organs  of  taste  and  smell,  have  no  more  escaped 
this  process  of  increase  than  have  the  other  sense  organs. 
The  vomero-nasal  organ  appears  to  be  a  kind  of  accessory 
receptor  for  smell  and  the  common  chemical  sense  is  ap- 
parently a  primitive  form  of  gustatory  organ.  But  in 
addition  to  these  subsidiary  receptors,  the  true  olfactory 
surfaces  as  well  as  the  gustatory  areas  are  not  homo- 
geneous, but  are  marked  by  local  receptive  differentiation. 
This  is  especially  well  illustrated  by  the  so-called  sense 
of  taste.  This,  as  has  already  been  shown  in  the  preceding 
chapter,  is  in  reality  not  a  single  sense,  but,  in  accordance 


with  Oehrwall's  opinion  (1901),  must  be  regarded  as 
generic  and  to  consist  of  at  least  three  and  probably  four 
senses,  namely  the  sense  of  sour,  of  saline,  of  bitter,  and 
of  sweet.  These  senses  are  really  distinct  and  separate. 
They  have  independent  receptors  and  give  rise  to  sensa- 
tions that  do  not  intergrade.  Their  association  under 
one  head  as  members  of  the  sense  of  taste  is  in  a  way  a 
misconception  due  doubtless  to  the  fact  that  in  ordinary 
activity  all  four  senses  are  commonly  in  operation  at 
once,  and  hence  acquire  a  certain  degree  of  functional  as- 
sociation. Taste  then  is  not  the  name  for  a  single  sense 
but  for  a  group  of  senses  and  it  is  likely  that  smell  is  of 
the  same  nature,  but  until  olfaction  is  better  understood, 
it  is  impossible  to  indicate  the  elements  of  which  it  is  com- 
posed. Thus  the  chemical  senses,  like  the  others  already 
briefly  enumerated,  show  the  same  tendency  to  increase 
in  number  as  they  become  better  known. 

4.  Classification  of  Receptors  in  General.  A  detailed 
investigation  of  the  chemoreceptors  leads  to  an  increas- 
ing multiplicity  of  elements  as  in  the  other  receptor  sys- 
tems, and  raises  the  question  of  what  constitutes  a  unitary 
sense  and  how  such  units  are  related.  When  one  or 
more  similarly  organized  receptors  are  excited  to  activity 
by  a  single  category  of  stimuli  and  give  rise  to  the  same 
kind  of  sensation  we  think  of  the  aggregate  as  a  sense. 
Thus  when  a  deforming  pressure  impinges  upon  any  part 
of  the  skin,  touch  receptors  are  stimulated  and  we  re- 
ceive a  uniform  impression  characteristic  of  the  sense  of 
touch.  Or  when  one  of  a  variety  of  sounds  falls  upon 
the  ear,  we  experience  hearing.  In  the  second  instance 
the  stimulus,  different  sounds,  is  open  to  much  greater 
variety  than  in  the  first  where  the  stimulus  is,  a  deform- 


ing  pressure,  and  in  a  corresponding  way  the  sensations 
in  hearing  are  much  more  diverse  than  those  in  touch. 
But  it  is  still  reasonable  to  regard  hearing  as  one  sense, 
for  its  various  stimuli  grade  into  one  another  as  its  sen- 
sations do.  With  taste  on  the  other  hand  such  is  not  the 
case.  The  acid  stimulus  as  an  external  agent  is  entirely 
distinct  from  the  stimuli  for  the  other  tastes  and  the 
sour  sensation  as  an  internal  state  does  not  grade  into 
other  gustatory  sensations.  This  separateness  in  stim- 
uli and  in  sensations  is  characteristic  of  the  four  kinds 
of  tastes  and  justifies  their  acceptance  as  separate  senses, 
a  division  that  is  not  permissible  in  hearing.  To  con- 
stitute a  single  sense  implies  a  reasonable  similarity  in 
stimulus,  receptive  mechanism,  and  sensation. 

But,  as  previously  pointed  out,  the  initiation  of  sen- 
sations is  a  function  of  only  a  limited  number  of  the 
human  receptors.  Many  of  these  organs  are  concerned 
with  activities  entirely  unassociated  with  sensation; 
hence  to  speak  of  them  as  representing  a  sense  seems 
somewhat  inconsistent.  If  the  term  receptor  is  an  im- 
provement over  that  of  sense  organ  because  of  its  free- 
dom from  implications  concerning  sensation,  it  might  be 
well  for  the  same  reason  to  substitute  some  other  term 
for  sense,  such,  for  instance,  as  recept.1  In  that  case  a 
recept  is  that  aggregate  of  action  that  occurs  where  the 
receptive  arm  of  any  reflex  arc  goes  into  normal  activity 
irrespective  of  whether  this  activity  is  productive  of  a 
sensation  or  not.  The  recept  then  includes  all  the  oper- 

1 1  am  fully  aware  that  this  term  has  already  been  appropriated  by 
the  psychologists  for  a  very  different  purpose,  but  as  they  have  taken 
almost  all  the  terms  in  the  language  for  their  own  use,  I  do  not  hesitate 
to  reappropriate  this  one  to  fill  the  present  need. 



ations  from  the  reception  of  the  stimulus  to  those  central 
changes  that  mark  the  entrance  of  the  impulse  into  the 
central  organ  including  the  production  of  a  sensation, 
if  such  occurs. 

Where  a  recept  is  concerned  with  sensation,  the  pro- 
duction of  this  state  may  be  regarded  as  its  final  step.  A 
sensation,  then,  is  an  activity  in  a  particular  region  or 
spot  of  the  central  nervous  organ  marking  the  central 
end  of  the  receptive  portion  in  a  reflex  arc.  Experience 
has  shown  that,  irrespective  of  the  means  by  which  this 
central  region  is  stimulated,  it  calls  forth  only  one  kind 
of  sensation.  This  in  a  way  is  a  restatement  of  the  mod- 
ern view  of  Miiller's  specific  energy  of  the  nerves,  for, 
according  to  this  principle,  however  a  particular  sense 
organ,  or  conducting  trunk,  or  nerve  center  may  be  stim- 
ulated, only  one  kind  of  sensation  results.  In  other 
words  the  character  of  a  sensation  is  not  determined  by 
peripheral  organs  but  is  strictly  a  central  affair  and  sen- 
sations are  different  not  because  of  the  different  sources 
of  the  incoming  impulses,  but  because  of  the  different 
central  spots  excited.  Since  the  anatomical  connections 
are  such  that  a  particular  receptor  always  leads  to  a 
special  central  region,  it  follows  that  such  a  receptor  be- 
comes thus  associated  with  a  given  sensation.  Hence 
where  sensations  occur  they  may  be  used  in  distinguish- 
ing receptors,  but  in  the  many  recepts  that  are  unassoci- 
ated  with  sensation  this  feature  naturally  cannot  be  called 
upon  as  a  means  of  discrimination. 

Although  numerous  receptors  are  in  no  way  concerned 
with  sensations,  there  are  no  receptors  that  are  not  ac- 
tuated by  stimuli.  Hence  the  stimulus  affords  a  more 
general  basis  for  discriminating  between  receptors  than 


the  sensation  does.  The  two  groups  of  chemoreceptors 
and  of  mechanicoreceptors,  already  frequently  alluded 
to,  show  how  fundamental  this  method  of  classification 
is,  for  these  two  groups  represent  the  two  well-recognized 
activities  of  our  material  surroundings  and  together  may 
be  put  in  strong  contrast  with  radioreceptors  such  as  the 
organs  for  heat  and  for  cold  and  the  eye,  all  of  which  are 
stimulated  by  radiant  energy. 

These  three  classes  constitute  the  fundamental  groups 
of  receptors  and  under  some  one  of  these  heads  every 
such  organ  should  find  its  place.  To  the  chemoreceptors 
discussed  in  this  volume  may  possibly  be  added  those 
on  the  wall  of  the  stomach  that  according  to  Carlson 
(1916)  have  to  do  with  appetite.  The  receptors  for  pain 
are  possibly  stimulated  by  the  chemical  action  of  ab- 
normal tissue  juices  and  the  endings  for  thirst  may  also 
depend  upon  some  such  form  of  activation  (Cannon, 
1918),  though  both  of  these  organs  may  belong  to  the 
group  of  the  mechanicoreceptors  (Muller,  1920).  To  the 
mechanicoreceptors  belong;  unquestionably  those  termi- 
nals that  are  excited  by  a  deforming  pressure  such  as 
the  receptors  for  touch,  for  pressure,  including  the  or- 
gans for  equilibrium,  and  for  hearing.  Very  probably 
pressure  is  the  stimulus  for  muscle,  tendon,  and  joint  re- 
ceptivity and  the  sense  of  fullness  in  cavities.  Pressure 
produced  by  the  contraction  of  the  muscular  walls  of 
the  stomach  appears  to  be  the  stimulus  for  the  hunger 
pang  (Cannon  and  Washburn,  1912).  The  lateral-line 
organs  of  fishes  and  amphibians  give  every  evidence  of 
being*  mechanicoreceptors.  Finally  radiorecepto-rs  are 
those  organs  that  are  stimulated  by  radiant  energy  such 
as  the  heat  organs,  the  cold  organs,  and  the  eye. 


To  ascertain  into  which  of  these  three  groups  a  re- 
ceptor falls  it  is  necessary  to  know  how  it  is  stimulated 
after  which  its  classification  is  simple  and  immediate. 

Although  a  grouping  of  receptors  based  upon  their 
stimuli  will  of  necessity  always  be  complete,  this  plan  of 
arrangement  is  not  entirely  devoid  of  difficulties.  Chief 
among  these  is  the  fact  that  the  same  stimulus  may  ac- 
tivate what  we  know  from  other  standpoints  to  be  differ- 
ent receptors.  Thus,  as  already  stated,  parabrombenzoic 
sulphinide  excites  sweet  receptors  as  well  as  bitter  ones, 
and  strong  material  vibrations  will  stimulate  the  organs 
of  touch  as  well  as  the  ear.  But  such  instances  appar- 
ently occur  only  between  closely  related  receptors,  for 
the  organs  for  sweet  and  for  bitter  are  so  closely  related 
as  to  be  regarded  by  many  as  belonging  to  one  category 
and  hearing  is  certainly  very  near  akin  to  touch.  Herrick 
(1918)  has  discussed  the  definition  and  classification 
of  receptors  and  has  urged  for  this  purpose  the  use  of 
four  criteria :  the  sensation,  the  stimulus,  the  sensory 
mechanism,  and  the  type  of  response.  In  his  opinion, 
however,  none  of  these  affords  a  wholly  satisfactory  basis 
for  discrimination  and  grouping,  operations  that  can  be 
successfully  carried  out  only  when  sufficient  information 
is  at  hand.  But  experience  scarcely  warrants  such  a 
conclusion,  for  it  is  much  more  difficult  now  to  discover 
the  interrelation  of  the  twenty  or  more  human  receptors 
with  all  that  is  known  about  them  than  it  was  to  make  a 
corresponding  statement  about  the  original  five.  The  real 
difficulty  lies  in  the  fact  that  the  numerous  receptors  that 
we  now  recognize  have  undergone  varying  degrees  of 
differentiation  and  hence  their  mutual  affinities  are  ex- 
tremely diverse.  This  brings  us  at  once  face  to  face  with 


one  of  the  problems  of  this  inquiry,  namely,  the  genetic 
relations  of  receptors. 

5.  Genetic  Relations  of  Chemoreceptors.  The  three 
sets  of  receptors  mentioned  in  the  last  section,  the 
chemoreceptors,  the  mechanicoreceptors,  and  the  radiore- 
ceptors,  are  more  than  mere  convenient  assemblages ;  they 
represent  natural  groups  of  organs  whose  relations  with- 
in each  group  have  a  certain  genetic  character.  This  can 
be  illustrated  by  the  chemoreceptors. 

Fia.  37. — Diagrams  illustrating  the  receptor  systems  of  the  following  verte- 
brate chemoreceptors:  a,  olfactory  organ  and  vomero-nasal  organ;  b,  organ  of  the 
common  chemical  sense;  c,  gustatory  organ.  After  Parker,  1912. 

If  the  structure  of  the  several  vertebrate  chemore- 
ceptors is  compared,  it  will  be  found  that  they  present 
three  types  of  organization  (Fig.37).  These  types  can 
be  best  appreciated  from  the  standpoint  of  their  constit- 
uent neurones.  In  the  olfactory  and  vomero-nasal 
organs  the  neurones  have  cell  bodies  in  the  receptive 
epithelium  and  their  axons  extend  as  nerve-fibers  from 
these  bodies  into  the  central  organ.  In  the  common  chem- 
ical organs  the  receptors  are  free-nerve  terminations  in 
the  mucous  epithelium  of  the  mouth,  the  nose,  the  eye  and 
other  such  apertures,  from  which  axons  provided  with 


deep-seated  cell-bodies  extend  into  the  central  organs. 
Finally,  in  the  gustatory  organs  the  taste-buds  are  com- 
posed of  receptive  epithelial  cells  that  are  in  synaptic 
relations  with  nerve  terminals  essentially  like  free  end- 
ings from  which  axons  with  deep-seated  cell-bodies  pass 
into  the  central  organs.  These  three  types  of  structure 
include,  so  far  as  is  known,  all  the  vertebrate  chemorecep- 
tors.  To  a  common  stimulus,  like  ethyl  alcohol,  the  ol- 
factory type  has  been  shown  to  have  by  far  the  lowest 
threshold  followed  in  order  by  the  gustatory  and  the  com- 
mon chemical  types  both  of  which  are  near  together  in 
this  respect. 

When  these  three  types  are  compared  with  the  recep- 
tors of  other  animals,  it  is  seen  that  the  olfactory  type 
reproduces  almost  exactly  that  found  in  the  skins  of  many 
invertebrates,  and  that  the  other  two  types  are  character- 
istically vertebrates.  The  integument  of  animals  even  as 
simply  organized  as  sea-anemones  is  rich  in  receptive  cells 
that  reproduce  in  almost  eveiy  detail  the  conditions  of 
the  vertebrate  olfactory  neurones.  Not  only  do  these 
lowly  organized  forms  show  this  structural  similarity  in 
their  integumentary  cells,  but  they  are  known  to  be  so 
responsive  to  minute  amounts  of  material  wafted  from 
distant  food  through  the  water  to  them  that  they  have 
been  for  a  long  time  past  credited  with  olf action  (Pollock, 
1883).  Thus  the  vertebrates  olfactory  epithelium  and  the 
integument  of  aquatic  invertebrates  are  strikingly  alike. 
It  is  more  than  probable  that  the  vertebrates  have 
descended  from  ancestors  whose  skin  was  an  epithelium 
like  that  on  the  exterior  of  a  sea-anemone  and  that,  as 
this  skin  thickened  over  most  of  the  body  to  give  the 
necessary  protection  to  the  slowly  metamorphosing  ani- 


mal,  the  future  olfactory  region  remained  unchanged  and 
thus  retained  its  original  invertebrate  character.  This 
region  became  the  olfactory  epithelium  of  the  developing 
vertebrate,  the  most  primitive  chemoreceptor  in  this 
group  of  animals. 

The  organs  next  in  this  series  were  the  common  chem- 
ical receptors.  The  neurones  for  these  organs  were 
differentiated  from  the  neurones  of  the  primitive  inverte- 
brate skin  by  a  central  migration  of  their  cell-bodies  till 
they  became  part  of  the  spinal  ganglia  and  thus  left  in  the 
integument  free-nerve  terminations  as  receptors.  This 
type  of  chemoreceptor  is  found  generally  in  the  skin  of 
fishes  and  amphibians  and  in  the  mouths,  nasal  chambers 
and  other  moist  cavities  of  the  air-inhabiting  vertebrates. 

The  third  and  last  type  of  the  vertebrate  chemorecep- 
tor is  the  gustatory  organ.  In  this  type  the  conducting 
neurone  presents  exactly  the  condition  met  with  in  the 
common  chemical  receptor  excepting  that  its  nerve  ter- 
minals, instead  of  being  free  in  the  integument,  are  asso- 
ciated with  epithelial  taste-buds.  This  type  of  receptor 
was  probably  derived  from  the  second  type  by  the  appro- 
priation of  taste-cells  from  the  integumentary  epithelium. 
Thus  the  three  types  of  vertebrate  chemoreceptors 
appear  to  be  genetically  related  in  that  the  olfactory 
organs  represent  what  may  be  called  the  first  generation, 
the  common  chemical  the  second,  and  the  gustatory  the 
third  (Parker,  1912). 

But  within  each  type  much  detailed  differentiation 
has  taken  place.  It  seems  to  be  quite  impossible  to  ex- 
plain the  variety  of  olfactory  sensations  without  assum- 
ing a  differentiation  among  the  receptors  of  the  olfactory 
field.  In  the  common  chemical  sense  the  receptors  on 


the  moist  surfaces  of  the  eye,  judged  by  the  sensations 
they  give  rise  to,  are  distinguishable  from  those  in  the 
epithelium  of  the  mouth  and  of  the  nose.  But  this  special 
differentiation  is  best  seen  in  the  gustatory  organs.  Here 
three  and  probably  four  well  defined  senses  can  be  dis- 
tinguished, namely,  sour,  saline,  sweet,  and  bitter.  And 
though  separate  receptors  for  these  four  senses  have  not 
as  yet  been  distinguished  structurally,  their  functional 
separation  is  beyond  doubt. 

It  is  because  of  the  repeated  differentiations  that 
characterize  the  evolution  not  only  of  the  chemoreceptors 
but  of  the  other  groups  of  like  organs  that  a  classification 
of  them  or  even  a  simple  enumeration  proves  to  be  so 
unsatisfactory.  For  they  are  not  unitary  elements  that 
can  be  counted  like  the  fingers  on  the  hand  nor  are  they 
sufficiently  co-ordinated  to  make  classifications  easy  and 
natural.  They  are  like  the  whole  organism  itself  in  that 
they  exibit  that  kind  of  diversity  that  characterizes  evo- 
lutionary flux. 


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/. . 



Acetic  acid,  137 

Activators,  18,  21 

Adequate  olfactory  stimuli,  80 

Aducco,  V.,  et  U.  Mosso,  153,   156, 


Ageusia,   156 
Alcock,  N.,  33,  39 
Alcohol,    171 
Alkaline  taste,  134,  135 
Alkaloids,   141 
Allison,  V.  C.,  and  S.  H.  Katz,  52, 

55,  56,  88 
Althaus,  J.,  81,  88 
Amiurus,  116,  160,  173 
Ammocoetes,  104 
Amphioxus.    103 
Anosmia,  71,  72,  83 
Von  Anrep,  B.,   153,   162 
Anton,  W.,  92,  100 
Appetite,    179 

Arnstein,   C.,   29,   39,   121,    122,   128 
Aronsohn,  E.,  57,  58,  60,  63,  70,  81, 

83,  85,  88 
Asai,  T.,  33,  34,  39 
Auxogluc,    144 

Babuchin,  A.,  29,  39 

Backman,  E.  L.,  60,  61,  77,  88 

Baginsky,  B.,  125,  128 

Baglioni,  S.,  64,  68,  88 

Ballowitz,  E.,  31,  39 

Barbus,  64 

Barral,  F.,  et  A.  Ranc,  162 

Basal  cells,  29,  30,  119 

Beaunis,  H.,  155,  162 

Becker,   C.   T.,   und  R.   O.   Hertzog, 

138,  162 

Bedford,  E.  A.,  39 
Beer,    T.,    A.    Bethe,    und    J.    von 

Uexkiill,  21 
Benzoin,  70 

Bethe,  A.,  17,  21,  22 

Bidder,  F.,  48,  87 

Bishop,  J.,  43,  88 

Bitter  taste,  140,   150 

Blakeslee,  A.  F.,  72,  88 

Blaue,  J.,  34,  39 

Blix,  M.,  175,  184 

Bowden,  H.  H.,  39 

Braeuning,  H.,  108 

Broman,  I.,  99,  100 

Brookover,  C.,  98,  100 

Von  Brunn,  A.,  28,  29,  31,  34,  35,  39, 

96,  97,  98,  100 
Bulbar  fibers,  123 
Bunzel,   R.,    147,   164 

Cannon,  W.  B.,  179,  184 

Cannon,  W.  B.,  and  A.  L.  Washburn, 
179,  184 

Carlson,  A.  J.,  179,  184 

Castronovo,  A.,  29,  33,  34,  40 

Catfish,  65 

Chemical  relations  of  odors,  76 

Chemoreceptors,   169,   175,    176,    179 

Chorda  tympani,  126 

Chumming,  68 

Circus   movements,   67 

Classification  of  receptors,   176 

Cloquet,  173 

Cocaine,  153 

Coelenterates,  19 

Coghill,  G.  E.,  106,  108 

Cohn,  G.,  78,  88,  141,  144,  162 

Cold  organs,  179 

Cole,  L.  W.,  103,   107,   108 

Common   chemical  organs,    181 

Common  chemical  sense,  102 

Comparative  distribution  of  taste- 
buds,  115 

Comparative  physiology  of  taste,  160 

Component  theory  of  taste,  152,  158 


188  INDEX 

Concha?,  23,  27,  38  Gawrilenko,  A.,  93,  100 

Copeland,  M.,  66,  68,  88  Gemmal  fibers,  123 

Crozier,  W.  J.,   103,    104,   107,   108,  Genetic  relations  of  chemoreceptors, 

138,   139,   158,   162,   163  181 

Gushing,  H.,  125,  126,  128  Gertz,    H.,    147,    163 

Glaser,  0.,  71,  89 

Diemyctylus,  68  Gley,  E.,   162 

Disse,  J.,  34,  36,  39,  40  Gley,  E.,  et  C.  Richet,  141,  163,  170, 

Distribution  of  taste,   148  185 

Dogfish,  64,  173  Glucophore,    144 

Dogiel,  A.  S.,  33,  40,  115,  123,  128  Goldfish,  63 

Drasch,  0.,  125,  128  Golgi,  32 

Drugs  and  taste,    152  Goldscheider,   A.,   und   H.    Schmidt, 

Duges,  A.,  43,  88  151,  163 

Durand,  A.,  48,  60,  68,  88  Graber,  V.,  15,  22 

Durrans,  T.  H.,  88  Graberg,  J.,  110,  114,  116,  118,  120, 

Duval,    134,    163  128,   129 

Grassi,  V.,  und  A.  Castronovo,  29, 

Von  Ebner,  V.,  116,  128  33>  34;  40 

Ecker,  A.,  28,  40  Greenberg,  D.,  69,  91 

Eckhard,  C.,  28,  31,  40  Group  of  chemical  receptors,  174 

Edgeworth,    152  Gustatory  chiasma,    125 

Ehrlich,  P.,  29,  40  Gustatory  contrasts,  156 

Electrical  stimulation  of  taste,  147  Gustatory  nerves,  123 

Eschricht,  D.   F.,   43,   89  Gustatory  nerve  fibers,   127 

Ethmoid  cells,  26  Gustatory  organs,  110,  182 

Extrabulbar  cells,  120  Gustatory  senses,   158 

Eye»    179  Gustatory  stimuli,    133 

Gymnema,  152 

Fischer,  E.,  und  F.  Penzoldt,  53,  54,  Gvmnemic  acid,  153 

56,  89,  184 

Flourens,  M.  P.,  174,  184  Hahn,  R.,   132,  164 

Foliate  papillae,  113  Haller,  B.,  73,  115,  129 

Fontana,  A.,   153,   163  Hamlin,  H.  E.,  100 

Franke,  G.,  45,   47,  89  Hammerhead  shark,  67 

Free-nerve  endings   in  olfactory   re-  Hiinig,  D.  P.,   132,   149,  163 

gion,  34  Harvey,  R.  B.,  138,  163 

Frentzel,  J.,  156,  163  Haycraft,  J.  B.,  57,  77,  87,  157,  162 

Von  Frey,  M.,  54,  89,  135,  163,  175,  Hearing,    176 

185  Heat  organs,  179 

Frolich,   R.,   44,   86,   89  Heidenhain,  M.,   113,   115,   116,   117, 

Frontal  sinus,  26  119,    129 

Fungiform  papilhe,  112,  150  Heiderich,   F.,    Ill,    114,    129 

Fusari,  R.,  et  A.  Panasci,  121,   128  Henle,  J.,  136,   163 

Kenning,  H.,  60,  68,  69,  74,  75,  76, 

Garman,  S.,  89  78,  79,  80,  82,  87,  99,  100,  158,  163 

Gaupp,  E.,   100  Henry,  C.,  163 

INDEX  189 

5elVT'iL''  I41'/55'   163  Kahlenberg,  L.,   135,   136,  137,   138, 

Herhtzka,    A,    135,    140,    142,    153,  139,  140?  164 

154,  163  Kallius,  E.,  31,  34,  40,  100,  116,  129 

Hermann,  F.,  Ill,  118,  119,  120,  129  Kamon,  K.,  34    40 

Herrick    C.  J.,  106,    108,    115,    124,  Karpman,  B.,  49,  91 

129,  160,  161,  163,  172,  175,  180,  Kastle,  j!  H.,  154,  164 

Hertzog,  R.  0,  138,  162  g^  **  ,f  ,f  56>  88        . 

Heymans,  a,   156,  163  ^  £  £  ?„,  136,  139,  140, 

Hisiology  of  vomero-nasal  organ,  96  £3'  }£  ^   153'  155'   157'  158> 

'  135'  139'  Kieso^  F,  und  R.  Hahn,   164 

A        11,    129 
Hofmann,   F,   und'l,   Bunzel,    147,      ^^  153,°?  64 

Honigschmied,  J.,  124,  131,  155 

Hooper,  D,  153,  164  Krause,  W.,  28,  40,  114,  126,  129 

Howell,   W.   H,   and   J.   H.  Kastle,  Kremer>  J-  H.,  62,  89,  157,  164 

154,  164  Von  Kries,  J.,  175,  185 
Humboldt,    146 

Hunger,  179  Landacre,   F.  L.,   128,    129 

Huyer,  C.,  78,  89  Larguier  des  Bancels,  J.,  61,  87,  162, 

173,  185 

Inadequate  gustatory  stimuli,  145  Larsell,  0.,  100 

Inadequate  olfactory  stimuli.,  80  Latency  of  taste,  154 

Innervation  of  taste-buds,  120  Lateral-line  organs,  15,  179 

Insipidity,    135  Von  Lenhossek,  M.,  34,  40,  97,  98, 
Inspiration,  48  100,   118,   120,  121,   123,   129,  130 

Integumentary  sense,  175  Leydig,  F.,  16,  115,  117,  130 

Intermediate  zone,  32  Lingual  nerve,  125 

Interrelation  of  the  chemical  senses,  Linnaeus,    73 

167  Location  of  taste,  132 

Intrabulbar  fibers,   122  Loeb>  J->  62>  89,  108 

Intragemmal  fibers,   122  Loeb'  R-  F->  62>  89 

Intragemmal  spaces,  120  Loven,  C.,  110,  117,   130 

Irritants,  44  Lubbock,  J.,  15,  22 

Lucas,  K.,    13 

Jacques,  P.,  121,   129  Luciani,  L.,  68,  87,   158,  162 
Jacobson  cartilage,  98 

Jagodowski,  K.  P.,  32,   33,   34,   40,  McCotter,  R.  E.,  94,  101 

62,  89  Magendie,  F.,  43,  89 

Johnston,  J.   B.,   115,    129  Marchand,  L.,   162 

Jourdan,    E.,    15,    22  Maxillary  sinus,  26 



Mechanicoreceptors,  179 
Merkel,  P.,  118,  130 
Metallic  taste,  134 
Meyer,  S.,  125,  127,  130 
Von  Mihalkovics,  V.,  99,   101 
Minimum    olfactory    stimulus,   49 
.Merrill,  A.   D.,   33,  34,  40 
Mosso,  U.,  153,  156,   162 
Miiller,   Johannes,   68,    152,   178 
M filler,  L.  R.,   179,  185 
Munch,  F.,  115,  130 
Myers,  R.  G.,  144,  145,  164 

Xagel,  W.,  48,  63,  64,  67,  76,  87, 
89,  103,  109,  156,  158,  162,  164 

Nasal  cavities,  23 

Nasal  membranes,  26 

Nasolacrimal   duct,   26 

Nerves  of  ol faction,  42 

Nerve  terminals  of  common  chem- 
ical sense,  104 

Nettesheim,  K.,    143,    165 

Neurones,   181 

Neutralizing  odors,  85 

Newt,  68 

Odiometer,  52 

Odor  mixtures,  83 

Oehrwall,    H.,    135,    150,    151,    157, 

158,  160,  164,  176,  185 
Oertly,   E.,   and   R.   G.   Myers,    144, 

145,   164 

Olfaction   and   radiation,   62 
Olfaction   in  fishes,  63 
01  faction   and   solvents,   60 
Olfactometer,   50 
Olfactory  acuity,  53,  77 
Olfactory  cell,  30 
Olfactory   cleft,   25 
Olfactory  epithelium,  27,  28 
Olfactory  fatigue,  69 
Olfactory  flagella,  32 
Olfactory  hairs,  31 
Olfactory  nerve,  35,  42 
Olfactory    nerve    fibers,    29 
Olfactory  organ,  23,  36,  181 
Olfactory  organ  of  fishes,  37 

Olfactory  potency,   77 

Olfactory  prism,  75 

Olfactory  reflexes,  86 

Olfactory  sense  buds,  34 

Olfactory   stimulus,   57 

Olfactory  vesicle,  31 

Olmsted,  J.  M.  D.,  68,  89,  127,  130 

Organ   of  Jacobson,  92 

Osmophoric  groups,  78 

Pain,  179 

Panasci,  A.,  121,  128 

Papillae  of  tongue,  111 

Parabrombenzoic  sulphinide,  154 

Parker,  G.   H.,  22,   66,  89,   90,   103, 

104,   106,   109,   161,   164,   173   183, 

Parker,  G.  H.,  and  E.  M.   Stabler, 

55,   141,   143,   165,   171 
Parker,  G.  H.,  and  A.  P.  Van  Heu- 

sen,  148,  165 
Passage  of  air  through  nasal  cavity, 


Passy,  J.,  54,  55,  77,  90 
Paulsen,  E.,  45,  46,  47,  48,  90 
Pawlow,    J.    P.,    86 
Penzoldt,   P.,   53,   54,   56,   89 
Peribulbar  fibers,   122 
Perigemmal    fibers,    122 
Peter,   K.,   40 

Physiology  of  gustation,   132 
Physiology  of  ol  faction,  42 
Picht,  P.,  43,  90 
Pier  cells,  118 
Tiutti,  A.,  133,  165 
Pollock,   W.   H.,    182,    185 
Polymorphic  cells,  33 
Ponzo,   M.,    Ill,    130,    165 
Potassium    chlorate,    156 
Prins,  H.  J.,  90 

Qualities  of  odors,  72 
Qualities  of  tastes,  134 

Radioreceptors,    179 
Ramon  y  Cajal,  S.,  29,  34,  40,  97, 
98,  101, 



Ranvier,  L.,  118,  120,  125,  127,  130 
Read,  E.  A.,  40,  97,  98,  101, 
Recept,  177 
Receptors,  18 
Reflex  action,  16 

Retzius,  G.,  29,  31,  34,  40,  41,  97,      g  143 

"I/~\T         i  i  f\        i  n  i         1  r»r*        i  c\  o        io/\  O         ' 

Sternberg,  W.,  87,  134,  154,  162,  165 

Stich,  A.,  134,  165 

Stimulation  of  chemoreceptors,   167 

Stovaine,  153 

Substances  with  two  tastes,  153 

101,  119,  121,   122,   123,   130 
Reuter,  C.,  72,  90 
Ribot,   T.,    185 

Richards,  T.  W.,  136,  137,  165 
Richet,   C.,    141,   163,   170 
Risser,  J.,  68,  90 
Ritter,  147 
Rod  cells,   118 
Rollett,  A.,  72,  90 
Rosenthal,  J.,  146,  147,  165 
Rubaschkin,  W.,  35,  41 

Saccharine,   142 
Saline  taste,  139,  150 
Sandmeyer,  W.,  125,  127,  130 

Sulzer,   145 
Supporting  cells,  117 
Sustentacular  cells,  29,  30 
Sweet  taste,   142,  150 
Symington,   J.,    101 
Systems  of  odors,  73 

Tadpole,  68 

Taste  alteration,  155 

Taste  compensations,   157 

Taste-bud,   110,    115 

Taste  cells,   117 

Taste   mixtures,    157 

Thirst,   179 

Sarasin,    P.,    und    F.    Sarasin,  100,      Thorns,  H.,  and  K.  Nettesheim,  143, 

101  165 

Schaeffer,  J.  P.,  24,  25,  41  Touch,    176 

Schiff,  M.,  43,  90,  134,  165  Tourtual,  C.  T.,  57,  90 

Schirmer,  R.,  155,  165  Trigeminal  nerve,  42 

Schmidt,   151  True  odors>  44 
Schultze,  M.,  28,  29,  30,  31,  32,  41,      Tuckerman,   F.,   110,    111,    115,    131 

58,  90,  175,  185  Von  Uexkiill,  J.,  64,  90 

Schulze,  F.  E.,  115,  130  Urbantschitsch,   V.,    165 
Schwalbe,  G.,  110,  114,  117,  118,  130      Valentin,  G.,  43,  53,  81,  85,  90,  91, 

Sense  organs,  13,  18,  21  134,    165 

Seydel,    O.,    100,    101  Van  Dam,  C.,  51,  91 

Sheldon,  R.  E.,  64,  66,  90,  103,  104,      Van  der  Stricht,  O.,  31,  41 

105,   109 

Sherrington,   C.  .S.,    172,    185 
Shore,  L.  E.,  149,  153,  165 
Sinuses,   25 
Smell  and  taste,  173 
Sour  taste,  136,  150 
Specific  energy  of  nerves,  152,  178 
Sphenoidal  sinus,  26 
Sponges,    21 
Stabler,  E.  M.,  55,  106,  141,  143,  165, 


Stahr,  H.,  110,  111,  131 
Steiner,  J.,  64,  90 

Van  Gehuchten,  A.,  35,  41 
Van   Heusen,   A.   P.,   148,    165 
Vaschide,  N.,  58,  60,  91,  162 
Vastarini-Cresi,  G.,   125,   131 
Vomero-nasal    organs,    92,    181 
Veress,  E.,  58,  59,  60,  81,  91 
Von   Vintschgau,   M.,   87,    131,    134, 

162,  165 
Von  Vintschgau,  M.,  und  J.  Honig- 

schmied,  124,  131,  155,  165 
Volta,  146 

Washburn,  A.  L.,  179,  184 
Weber,  E.  H.,  57,  58,  59,  91 

192  INDEX 

Winslow,   C.-E.   A.,    and   D.  Green-      Zenneck,   134,   166 

berg,  69,  91  Von  Zevnek,  R.,   147,  166 

Von  Wittich,  W.,  154,  165  _                         „ 
Woodrow,  H.,  and  B.  Karpman,  49,      Zu<*erkandl>  E-   "1 

91  Zuntz,  N.,  156,  166 

Wundt,  W.,  134,  166  Zwaardcmaker,    H.,   45,   47,    48,   50, 

Von  Wyss,  H.,  114,  131  51,  57,  58,  59,  62,  70,  72,  73,  76, 

Zander,   R.,   124,    131  77,  85,  87,  91,  162